epoxy grout in contact with regular sanded cement grout
It's OK to do that. The epoxy grout is great for bathrooms because it doesn't stain, is virtually waterproof and it's not affected by the cleaning supplies you'd usually find in a bathroom. The installers need to do the epoxy grout first and let it set up. It needs to stay in the bathroom because the sheen will be different than the regular grout so you don't want it to be worked into a hallway. Once the epoxy group has hardened, but not completely, they can use the regular grout right up against the epoxy group which will not absorb any moisture from the regular grout... .enjoy your new tile.