We have two projects basically, each of 2 these main projects have sub projects which they are comprised of. What we have in SVN is currently 1 repo like this:
/SVN-REPO1/MainProj1/
-trunk
-branches
-tags
Should I create a completely new repository altogether and just put MainProj2 inside of its own? Or should I put MainProj2 inside of SVNREPO1?
I think it depends on the relation between MainProj1 and MainProj2. If they are very related, then use the same repo.
Creating a different repo for each project is handy for tracking history, merging and branching independently. You can still do it if both projects are in the same repo, but having them on different repos just makes it simpler.
Think of maintainability too. Maintaining this repository, with two main projects with sub projects, could become more complex as the projects grow.
Unless both projects were very related to each other, I would go for separate repositories.