Save time and disk space by using Bazaar's repositories.
Starting from scratch
Create a new repository and create a branch within it. I'm going to use MAAS here, but you should use whichever project you're interested in here, provided it manages its code with Bazaar.
It doesn't matter what you call that branch, or what branch you start with. The essence of why this is useful is because the history of every branch created within the repository directory is shared with all the other branches. This means that starting a new branch is quick — just a copy of the working tree, with a little additional disk space used for branch history:
If you have a number of branches already, you'll want to migrate.
Create the repository in the parent directory, then reconfigure the branches:
Repeat that for the rest of your branches. The first reconfigure will take some time but the second and subsequent will be quick.
In other respects these branches / working-trees behave the same as their standalone cousins.