The problem: I often have to update two or more repositories: One for the project I'm working on and one or more libraries. Is there a way to "chain" the updates, so that every time I update project X, library Y gets updated, too?
I know, this is an unusual question, because I guess things like that would be part of a build process, but as the things are now at my workplace, this is not possible (yet).
I'm using SVN 1.5 together with TortoiseSVN.
Sounds like something you should be able to accomplish using svn:externals. Essentially what it does is to "link" in an external (separate) svn-repository into a directory in your working copy.
Tortoise has excellent support for externals allowing you to select whether or not to export externals as a part of an WC-export or not. You can find a quick introduction to TortoiseSVN and externals at http://justaddwater.dk/. It might be a little out of date but it should get you started. The tortoise svn guide also offers some guidance.