A.V.
Bathe them at different times and keep it fast and simple with your 2 yr old. She is not happy in the tub, so perhaps instill a "Hokey Pokey" routine with her and make it as quick as possible. There was a phase my DD went through where she hated her bath (specifically washing her hair) and I did "standy up baths" that were mostly sponge baths, but it got her hosed down after a day at the park or whatever. If their father is home, consider washing the 2 yr old first and handing her off for books and pjs or whatever the routine is, while the 4 yr old gets her tub time and then put them to bed half an hour or so apart.