If the basic topic of the two forums are the same or relevant then I would merge together unless there might be issues for example:
One forum is very polite and diverse but the other one is more aggressive and with male oriented sexist banter I would think twice before merging as you may lose some of the members who don’t appreciate that stuff.
However from an SEO point of view merging may well be better as you are. Cutting out some competition but hard to say. You end up with a bigger forum that could be more impressive for sponsors if you are commercial. If there are other competitive sites you may do well by growing