Well, my parents came from moderately strong religious backgrounds, my father Catholic and my mother Jewish. They had different experiences with their faiths, though. Both attended private religious schools as kids, where my Dad said he lost most faith in the institution of the church, but seems to have kept the basic monotheistic belief and the cultural foundation that it rests on. He recalls often days where his teachers, Brother Paul and Brother Francis or some such, would have to be dragged into class dead-drunk while the other made excuses to the children for him: "Oh kids, Brother Francis is just...he's sick today." On the other hand, my Mom followed most of her family's orthodox beliefs and customs through high school level and graduation. She believes in a style slightly more liberal than them, but there was no loss of trust in it on her part, so she grew up solidly Jewish. She didn't have many friends, but had great grades, and graduated one year early.
They met when they went to the same university, where my Dad was struggling to pass, and my Mom was helping him in what ways she could. The two of them fell in love, and soon after graduation began thinking about marriage. Their families were quite opposed to the idea, thinking the other culture/upbringing too alien and closed-off in their ways. The families didn't trust each other. Legend has it that both my grandfathers hated each other (well, they are both very stubborn men), so when they met and shook hands at the wedding, it was as though the Earth had moved.
Over the years and decades, my Dad's family accepted their differences gently, and respected my Dad's decision. My mother's family, however, never quite understood or forgave her for her "mistake," so every family can take this course differently.
They were both disillusioned enough with their families that, once married and thinking about starting a family, they moved away from the city in which they grew up, and then after having kids, moving even further away. Not the only reason for moving, but a factor that made them "okay" with parting with their past, the city they had known for their whole lives. Making a new home about a thousand miles away, they visited their relatives once or twice every year. My Dad's parents visited us also about once or twice a year, whereas my Mom's parents only visited us once in my life, and not while staying at our house--they stayed at a hotel and had us go to town to visit them. That was also, supposedly, to settle a score with my Mom's sister so that they could claim they visit us more than she does (which is precisely "never").
So how did they reconcile their beliefs? What my Mom has told me is that she married my Dad on the condition that their children be raised in Jewish ways, and he had to be okay with that. He was, since he had no strong attachment to his Catholic faith, and in his words, "It was more important to me that we raise kids who know right from wrong. As long as we have that, the rest falls into place." That seems to be the cornerstone of the religious part of their marriage. We indeed went to synagogue weekly and learned the prayers and customs, not practicing Christian ones, even after moving to and growing up in a social environment that was overwhelmingly Christian. My Dad went with us, even though he didn't know the words. It's strange, trying to remember it....something I wasn't paying keen attention to as a young child; I guess he mostly sat there and listened, and if a song was really catchy, he might pick up on how it goes, but not likely when it's in a foreign language. He would read along when we got to spoken English sections, though. He got involved in what small ways he could, for example, playing guitar at Sunday classes to help the Rabbi lead the kids in song. So he was invested in this, because it was part of how his kids were being raised, even if he wasn't accustomed to some things and had to follow along quietly rather than fully participate (due to lack of familiarity with how things go). Even when we didn't go to synagogue, my Mom led a small prayer and service for just us (Dad, Mom, my sisters and myself) in the kitchen at home. My Dad sang along with the English parts, and participated in "what's basically Communion, anyway," as he has said before.
Not only was my Mom less Orthodox in belief and more flexible in her practices than most of her family, but because of the fact that they didn't live in an area that was largely Jewish (the way my Mom had when growing up), circumstances would have made it difficult to be Orthodox anyway, so our upbringing was more liberal than my aunt, uncle, cousins, and grandparents were comfortable with. (For example, their belief that you're not supposed to drive on the Sabbath...and yet where we grew up, the closest synagogue was 20 miles away. Not to mention, a little more difficult to eat Kosher, but not impossible.)
In short, my Dad supported my Mom's view, and my Mom led the way, in the religious side of our upbringing. Their marriage lasted 22 years, and as far as they've said, and from what I can tell, the divorce had nothing to do with differences of belief. It had to do with a growing emotional distance in their relationship, not spending as much time together, eventual communication breakdown, etc. Once my Dad started dating again, and now that's he remarried to a Christian (Baptist), I notice that he goes with her to church, and I don't know whether that's him reconnecting with the Christian faith of his past, or just moulding to his wife's (and local society's) way of life. Probably a little of both. My Mom, too, started dating and did not limit herself to Jewish men, but again, reserves the right for herself and us, her children, to practice our religion separately. Not that we live with her anymore, which makes part of that easier to maintain with a husband/fiance, but when we visit her for holidays and whatnot. She likes to share her religion with her fiance and his family and anyone who's interested, she loves to get people involved, and her "stepdaughter" is welcome by them to experience both sets of holidays. As am I and my sisters, when we go to my stepmom's, or my "stepfather's" family's Christmas/Easter/etc. parties.
One strange thing I didn't realize until I started writing all this was that I've never really known what my Dad's true beliefs are. I've never asked him, as that's not the kind of thing we do in my family, being a naturally private bunch, and he's never stated any one clear whole system out loud, except for what I have written above. But how did he truly feel about the whole thing through his marriage with my Mom? It's not clear, since he's never much talked about it. I also have the feeling that maybe not even he is sure what he himself believes. And once again, I realize as I grow up and get older just how quiet and private my father is.
Thoughts that never occurred to me in childhood and teen years.
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In your case, I could imagine a Muslim-Christian marriage working out if the definitions you give yourselves are flexible enough, which is perhaps easier in this day and age than it would have been in the past (to interpret religion loosely, that is). And I guess different customs would physically have to be performed separately (you could do both, though, of course, one after the other...the "separate" doesn't have to imply that you separate from each other in order to do these), but to me, I think that agreeing that what you two believe is essentially the same would probably be crucial. Common values. I wonder if that's what my Dad did...
I am glad that this maybe made you think about things in a different way than you did before. It's funny as I was reading, I was thinking about attending Shabbat dinner with my friends and learning the Motzi and Kiddish (in like... catchy phonetic fashion)! I feel like the type of person I am, being spiritual but interested in other faiths (as portrayed by my participation in friend's religious practices and my interest in researching/discussing my bf's faith...), it will be easy to be clear about what I believe and how it's compatible with other faiths' teachings. 
