Big No, No. From what I understand. They could catch the same thing from each other in a soak that they could catch being housed together. Probably faster in a soak, should one pee or poop
HUGE no no! Feces is the most common way to transfer diseases and parasites. You certainly don't want them bathing in it. The whole point of keeping different species separated is to PREVENT feces contact. I only soak tortoises together if they live together in the same enclosure. And even then only small ones.