In regards to the Asari breeding. Whichever Asari wants to carry the kid and be the majority caregiver, is the one who has it and does the meld, it's as simple as that I think.They would probably just impregnate each other if they both wanted a kid that represented their specific genes. Since the one mother contributes both sets of genes, and the other person is just used to randomize it pretty much. I've yet to hear of any Asari "marriages" in game so I'm also going to assume that raising a kid is a more solo affair with an occasional visit from the 'father'. Can't say that for certain though, we really only have Liara's parents as a example.
That actually bugs me even more then the breeding thing. All hints in game point to Asari only haveing one meld partner their entire 1000 year life, yet we don't see any married Asari or anything like that. They are ALWAYS solo. Just seems odd.