Showers, sinks and toilets all connect to a main stack vent (usually 3") in modern home construction, so it isn't a modern code issue. My service manual for 1979 models shows combined grey and black tank venting, so it is done on newer models. I think if you were to bridge the two with a crossover high enough to prevent cross- contamination, you'd be okay.
Cameron & the Labradors, Kai & Samm
North Vancouver, BC
Live! Life's a banquet and most poor suckers are starving to death! - Mame Dennis