Do more peppers mean more heat?
To put it simply: if you increase the amount of capsaicin per bite of food, you'll make it hotter.

So if we're talking about just a sauce that's basically pure peppers, then yes, the mixture of a very hot pepper and a more mild pepper will be somewhere in between the two, and the addition of the mild pepper dilutes the sauce - a spoonful of it will not contain as much capsaicin.

But if we're talking about a dish that's mostly other things, with ghost pepper for heat in the sauce, and on top of that you add another pepper to the sauce, then yes, the other pepper will add heat - there'll be more capsaicin in each bite. Will it be noticeable? If it's a banana pepper, no - it's orders of magnitude milder, so you'll never notice. If it's a habanero, probably - it's not that much more mild than the ghost pepper.

Other peppers may well add great flavor, though. There's quite a variety among all the chilis, and it's perfectly reasonable to use half a dozen different ones in a single dish just to get the nice full flavor you want. This is still true if you're making a really hot dish. That said, if you're talking about "the hottest foods" and ghost peppers, it sounds like your goal is really just to make something really hot, not something you can actually taste - you can definitely get plenty of heat from other peppers. If you're using the hottest possible peppers, it's probably either a gimmick, or the whole point is to burn your tongue off so you can't taste anything else anyway.

This is of course all assuming you're actually using the whole pepper, in order to get all the heat from it. If you're not, then... why are you bothering with ghost peppers? See Cos Callis' answer for more explanation.