Have you tried bath seats? They're not very AP but if you are there and vigilant I think they are OK safe. THey were a lifesaver for us at that age, where they could sit up.
I used 2 of them at the same time to bathe my boys. I used them from about 7 months til about a month ago, since they do better now in the tub sitting and playing. I have always bathed them together since they were about 7 months old.
In the seats I would get them both washed while they played with the toys floating around them, it was easier to rinse them off. Once they were both cleaned, I drained the tub, and I was by myself I had the diapers in there with me, pulled one out at a time and dried and diapered, and then the other one. If DF was around we just each grabbed a kiddo and dried and diapered.