At the moment my thoughts are:
Is a year really enough time to determine that you want to have children with these people?
Who will be the legal guardians?
How will you discuss things with the children?
What about when they are in school and have the others children on the playground asking questions?
I'm not against it, but I think there is a lot there for you all to discuss and think about. Alot of good and bad can come from this. It all needs to be considered before making such a decision.
My biggest concern is, why from the other partner first? I'm 27 my husband is 29, we've been together 8 yrs married almost 5, and we have no children. When the time comes for that, I want to have my husbands child.
It's a big step all around. I wish you luck in that decision, but for your sake and that of the kids, don't make it lightly.