I have two working copies of the same project, each switched to a different branch. Is it possible to merge local changes in one project to the other working copy?
You could create a patch on one working copy (svn diff) and apply it to the other one (patch).
http://ariejan.net/2007/07/03/how-to-create-and-apply-a-patch-with-subversion/
You can't merge two separate working copies, but there are some things you can do.
If the changes are made to existing files, the easiest way is to create a patch with svn diff, and then just apply that patch to the other working copy. Eg. in the first working copy you do:
svn diff > patch
And then you apply it in the other one:
patch -p0 < patch
And as usual you want to run patch with the --dry-run option to make sure it works first.
However if the branches don't have the same file layout, then you can't just take a patch and apply it. In this case what you need to do is simply just first commit the other local changes, and then use svn merge as usual