Mixing formulas!?!

mo

My daughter had diarrhea and was put on a soy based formula Prosobee and I don’t like it. ( idk why but I just don’t like it) she was on Gentlease and I want to transition her back to it and her pediatrician recommended mixing both formulas! I gave her an ugly look because they’re both very different formulas.. has anyone heard from their doctors about mixing formulas? How safe could it be?