I'll stop messing now. Two SFs together won't be that much of a major change providing their values are aligned. SFPs with Fi can sometimes have values that are completely different to what an SFJ may believe in, which may cause conflict.
You might have an idea on how you already communicate as an SFP. Personally I communicate my feelings through words and activity, which is fairly normal. With regards to how I pick up on other people's emotions. I don't believe I do that so much, however I am focused on my surroundings for the atmosphere. The atmosphere is completely dependant on what I think is happening, and the more experience I gain, the more accurate it becomes in guessing whether there is something wrong. Again this seems like really basic human behaviour, so I don't know whether it's any different from other temperaments.
Other SFs may be different but I don't think that it's possible to pick up the emotion, it's just a very strong case of how would that person feel if I were in their position with what I know about them. Thinkers can do this as well, however some of the ISTJs I've spoken to on Vent feel as if it takes too much effort to bother with and they'd rather focus on something else. Difference of priority? *Shrugs*
Note: Not all ITJs feel that it's a waste of time.