[Cruiser]

I'm installing black and grey water tanks (had none originally), and am trying to figure out how to route plumbing vents. There's a good place to run a vent pipe for the black tank, but not for the grey. I had hoped to run it up through a closet, but there isn't one close enough. My question: can I connect a vent pipe to the grey water drain line (sort of like house plumbing), and run it up to meet the vent pipe for the black tank, or do I need a separate vent pipe out of the grey tank? It's a 12 gal. tank.
Doug