Do you mean "Can you combine them in the same bottle?" or "Can you dose both in the same tank?", CLINTOS? Also, which bacteria?
Combining phytoplankton and bacteria in the same bottle: Probably not a good idea. One could potentially poison the other at high concentration.
Combining phytoplankton and bacteria in the same tank: I've never tried, but I don't see why you couldn't. Just be careful dosing them, unless you want to turn your tank pea green or hazy.
What changes? Now you're trying to culture multiple things in the same tank at the same time. Some of them will probably compete with each other. Others will end up feeding each other.
Are there any benefits? Maybe. I don't rightly know.