I agree with the above. Definitely NOT a good idea for many reasons.
A sponsor is more than someone to talk to - you can still be there and talk things through (including Steps stuff) when you want to. A sponsor needs to have worked the steps and know how to guide someone else through them.
My sponsor can get away with pushing me / saying things (that I need) in a way that would generally lead to disagreements if it was my other half as there's to much vested emotional interest there.
Plus you'd be missing out on the opportunity to get a new, unbiased perspective on things.
I really don't think being each others sponsors would be 'keeping it simple' by any stretch of the imagination.