A.G.
I take my babies right in the shower right from the start (as soon as the belly button falls off). Until last July my tub wouldn't hold water so that was not an option. I did use an infant tub at times but for a quick clean up I would take the baby in the shower with me get her claen and then hand her to Daddy who would dry and dress her while I finished in the shower. I think if you feel comfortable doing that then the sooner the better to get them use to it.
As for when a b/g twin set should stop bathing together I have no idea. I have all girls! A.