Though I'm about 75% sure this isn't do-able simply because no one has already done it, I guess I'll ask anyway: is it possible to create a mod so families could adopt 2 (or more) kids at once?
In my mind, which is completely unhampered by knowing how creating mods works, I imagine it being something like a cross between the mod that allows one to pick the gender of adoptees and the hack that allows one you pick if your sim gives birth to 1, 2, 3, or 4 babies. The only reason I think this might be possible is because of that screen telling you that there's a new baby on the way appears during both births and adoptions. But like I've said, I know nothing about hacking; my only attempt at creating a hack resulted in non-autonomous computers...that were also invisible!

Thanks for thinking about it.