That's a tough one. If it were a long term relationship, where the value in the images is derived from the shoot and print sales and not future stock, it might be worth considering. If either of you are distinctly different in the style you produce or are a better photographer, I wouldn't. It's not at all uncommon to freelance to busy studios for wedding work or portraits but that's usually hired out on a per shoot basis. I used to do this for busy studios. But to partner in a shoot? Hmmm. If you ran a really busy studio then you would both work the busy season but get time off without worrying about shutting down for the off season. You could go travelling or take time off and still see your studio operating. A big plus.