I'd suggest you to dump your current project, create a new one, load it there and then remove unneeded files from both. This way both will have full history and you can even recover deleted files on both.
Let's call your reposity MyProject which is located in /svn/repos/MyProject
Now you can checkout MyNewProject, remove files you don't want be in this half and commit. Then do the same with MyProject. If you fail, you'll have your reposity dumped in myproject.svn_dump.
To be extra safe you might want to create two new reposities.
I'd suggest you to dump your current project, create a new one, load it there and then remove unneeded files from both. This way both will have full history and you can even recover deleted files on both.
Let's call your reposity MyProject which is located in /svn/repos/MyProject
Now you can checkout MyNewProject, remove files you don't want be in this half and commit. Then do the same with MyProject. If you fail, you'll have your reposity dumped in myproject.svn_dump.
To be extra safe you might want to create two new reposities.
/
/projecta
/projecta/A
/projecta/B <-- should be moved to project B
svn move /projecta/B /projectb
The history on the file level will be kept intact for each file/directory