I believe that two people can become soulmates in the process of a lifelong relationship. But I think this applies to virtually any two people if they're committed and dedicated enough. I don't think it does happen too often, because mostly people just quit too easily, long before the stage when it's possible for them to truly know each other well enough.
And the idea that there is one special person "out there" who you'll know instantly is Right For You is just a crock of shit. You can't know anyone that well until you've lived with them at least a couple of decades. You might think you do, and tell yourself you do, because you want to, but reality's just around the corner.
A shrink once said to me (off the record) that soul mates are basically like siblings you're allowed to fuck - you can't get that close to anyone until you've lived a significant and formative chunk of your life expectancy at close quarters with them, sharing a room with them and with your lives inextricably linked with no option of exit from the situation. And you're still talking to each other, and haven't tried to kill each other for at least a year or two.
Anyone want his card?