Over the past month or so, we have changed our bath routine. We used to bathe them in the baby bathtub on the counter in our bathroom. We liked using it on the counter better than in the big bathtub, because our backs would get tired from bending over into the bathtub to wash two babies.
For the past few weeks, I've gotten into the big tub in our bathroom, and Jason will hand me each baby. I bathe them and then let them "swim" around a little bit in the bath while I hold their heads up. They love to kick the water and hit it with their little hands.
Last week, I bought a mat for the tub to add more traction and make it easier for them to sit up without sliding all over the place. Miles is getting close to being able to just sit in the bathtub without any assistance. Vivienne can sit fairly well, but I think it will be a few weeks before we can have her sitting in the bath by herself (still worried that she'll slip and bonk her little head).