Combining gray and black water
in the same tank violates USCG regs...it's illegal even for the two systems to share ANY common plumbing
, including vents. So that option is out.
However, it IS legal
to send gray water
and I strongly recommend you continue to do so, 'cuz managing a gray water tank (dealing with sludge and preventing odor) is a LOT more difficult than managing a black water tank. A sump is the closest thing to a gray water tank you'd want.
How many black water tanks
you'd need depends on how far apart the toilets are...best that the tank be not much more than about 6' from a toilet 'cuz 6' is about as far as bowl contents can move in the amount of time anyone will spend pumping a toilet or leave their finger on a flush button. That's not cast in concrete...there can often be work arounds in the plumbing
that provide an assist from gravity to get the flush all the way to the tank.