In my experience, the weed dealers aren't the same dealers as the guys moving the harder stuff. If you can get meth from a guy, he can probably get you coke too. But your weed guy probably can't get you either. At least, IME with NYC dealers.
Yeah, my weed guys over the years were just some fellow stoners. The most cross-promotion I saw was for mushrooms or other psychedelics but I feel like that fits the stereotype. Once in awhile they might "have something" but I'm would be a one-off thing.