The project files and the source control aren't coupled to one another. Any "branch" is going to include the entire source tree, not individual "projects." (The source control system in this case, SVN, has no knowledge of what a "project" is or how it differs from anything else. It's just a text file in the source tree.)
You should branch at the root of the entire codebase, essentially making the branch be a fully self-sustainable version of the codebase.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…