You could have it so both people have to add each other before there's any indication that either person added the other.

No extra work for person A, and the work for person B is just what person A had to do anyway.

This is mostly unusable for what should be obvious reasons.

I'm... not actually clear on what those reasons are? For the adder, the experience is exactly the same - the only difference is that there's no longer an adder and an addee - instead there are two adders.