That is such a hard age. I think I was still bathing ours one at a time until they were almost a year old because then they were a little more stable. I always worried about them tipping over and hitting their heads on the tub.
What I do now is rinse their hair with the washcloth. I try to start at the back where they don't notice the water running down and slowly move toward the front of their heads and to their foreheads, squeezing the water out of the cloth and sort of wiping the water and soap down. When i get to their forehead I put my finger under their chins and quickly tilt up their heads and squeeze some water on their hairline, wiping it back away from their face then quickly dab their face with a dry towel so that they don't start screaming. I guess, in other words, I try to rinse the water out without disturbing their playing, hoping they won't notice what I'm doing till I have to quickly tilt their chin up. And then I do that so quickly they go right back to playing. it works fine with Greenbean but I'm letting Sisi's hair grow out and the length requires more rinsing so she's not happy with that. When they get older, they might like wearing swimming goggles while you rinse their hair. My 3yo loves that. He asks me all the time to dump water over his head.
struggling mom to Gator ('03) Cha ('05) Sisi and Greenbean ('07) and the hubby child ('71)
"I'm sure we can handle this situation maturely, just like the responsible adults that we are. Isn't that right, Mr. Poopy Pants?" - The Naked Gun 2 1/2