After considering several options, our new inflatable tub is doing just the trick. It fits both girls, so I only have to run bath water once, and I can bathe the girls one right after the other - such a time saver (more time to splash around!). The girls are super comfortable and most of the time look like they are lounging poolside when they aren't lunging for bath toys which we also have room for in our new tub!
I can't wait until they can sit up on their own and play, I mean take a bath, in the big girls' tub. Bath time, I'm sure, will take on a whole new meaning.