I am not sure if you are confusing roasting with charring but both are the same basically, though roasting the pepper by charring the skin would be a more accurate nomenclature.
Roasting brings smokiness to the pepper and softens its bite. It allows you to remove skin more easily. When you char the outside of dried peppers in a dry skillet with spices, then soak, it allows you to remove the pith more constructively than tearing it open and it adds depth to the flavor.
For fresh peppers they become somewhat less spicy, for dried peppers it allows them a more expressive flavor (think of roasted sesame or pumpkin seeds or coffee beans for a comparison). Roasted peppers add a different dynamic than their raw counterpart and you should definitely look into the difference (if you enjoy chipotles end adobo you already have a taste for them without knowing it).
Many references indicate that stress on the plant effects the heat rating of the fruits produced. "Good" stress (usually people want to increase heat) is generally a reduction in water supply, carefully and aptly timed, and/or increased outdoor temperatures.
While appearing dated and non-authoritative, this site, also looks very accurate from my experience around growing chiles, and states that (as is common, referring to spice/heat as pungency):
...total pungency amount of the medium and mild chiles increase
dramatically when put under stress.
Normally, chile plants like summer weather (sunlight and warmth), and are not nearly productive in winter. What you're obtaining in winter months may be imported from a very different location, probably from a place on Earth where it was summer when harvested. As indicated in the above-cited article, the truly hot peppers don't get much hotter from stress.
Anecdotal addendum: fwiw & ime, chile plants grown in winter (the ones observed are nagas, tabasco, japone, jalapeños) aren't as productive and the peppers are not anywhere near as hot. So, when not grown in the proper conditions that a species expects - mostly enough daily sunlight or a long enough warm season, the heat in produced fruits can drop dramatically.
Best Answer
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.