I wish there was better documentation for this for Xcode 5 but I couldn\'t find anything decent describing best practices for working with workspaces, so..
In a workspac
This solution was intended for another problem, but maybe it fills your needs.
After adding your subproject to the workspace:
Now you are using the chosen library for your target. Repeat those steps for every target which needs this subproject. Hope that helps! For me it did because i could not manage to add dependencies to it before...
Greetz, Tomte