Combining black and Gray

I should have mentioned I have a macerator permanently installed so I never have to hook up a hose and don't have an end cap. I can see where doing this and dumping conventionally would be an yucky mess. But I don't ever need to touch my hoses so opening the valves is still a closed system unless I turn on the macerator. The macerator is essentially an automatic end cap. Matter of fact even when I have hookups I leave the gray open but it won't dump unless I hit the macerator switch. So this method wouldn't work with a conventional gravity drain but can't see why it wouldn't with my setup. The question is really about an issue with combining the tanks not actually dumping.