I have probably missed this, but can multiple mods be used at once if both mods have the same files in their xml folders? For instance, could I use the space realism mod and the venator full mod by placing their xml folders inside the main xml folder?
You never want to put their xml folders inside your main one. They would work only if they don't overwrite any common files(i.e, if only one uses factions.xml, for example), which is probably not true. You'd have to install one, then do whatever edits are necessary to implement the other.
to answer your question, no
To reply to both answers, poop.
but if you studied what they did in the mods, find out how they did it, you can make your own mod with how you want. or wait until they use each other ideas in an update