How to combine 2 standalone trees

Some while ago I took over a project new to me and not in any source control system. I wanted a simple setup, so I configured the project in bazaar using the 'solo' workflow model (a standalone tree, I believe). All the code below the 'workspace' directory is in bazaar. Later, I discovered that there were files in a sibling directory 'config' which I needed to track too, so as a quick measure I created a standalone tree here too. So I now have 2 separate projects /base/workspace and /base/config. However, it is a bit inconvenient having the trees separate like this, as ideally I would like to be able to issue a command like bzr commit from the /base parent directory to commit everything in both projects (obviously, this doesn't work - I get the error bzr: ERROR: Not a branch: "X:/base/".)

Is there any way to 'combine' these projects whilst preserving the change history?