my buddy and I speedrun games together and we were interested in running the Torch God% category, but found that there is no co-op categories for the "Terraria: Category Extensions" game on speedrun.com. any way that this could be possible? I know there are other players who would come attempt co-op CatExt% runs as well.

Yeah torch god doesnt have coop and I don't believe we plan to add it. Some categories do have coop though.

Torch God% doesn't say that you can't play co-op, so go for it! There are no category distinctions on the Category Extensions page :)