AFAIK it's not something you can do without running active scripts on every pawn (which is a bad thing). Apparel can't exist in the same slot, there's no mechanic to allow multiple things in the same location. Items have to be patched to occupy a different bodypart or layer, that's really the only way to do it currently. You could patch molotov belts to be one layer and grenade belts to be a different layer, but they can always only occupy that one defined slot once in game.

Packs Are Not Belts is really the best and only solution to the modded utility item problem AFAIK. There are creative ways to make a pawn able to do more though, like you can use biotech genes to give them fire or firefoam abilities which they can use in addition to another utility item.

You could also just bring molotovs with you in the caravan loadout and equip them if needed.