Trying to manage two babies in a tub when you only have two hands is tricky. They're as slippery as little fish in that watery environment. To help keep them secure, plastic laundry baskets make a great bath seats for older babies and young toddlers. They are the right size to contain their little bodies and keep them upright so they don't slide around in the tub. Use them with only a small amount of water in the tub. Use them only with babies who are old enough to sit up.
I haven;t tried this yet but am going to soon. It would be so much easier to bathe them both at the same time. I'll let you know how it goes. I just want to wait a few more weeks so they are both really stable sitting up in the tub.