There's no limit on how many repos your working repo talks to, so just
Code:
git remote add 2.6.38.stable git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-2.6.38.y.git
git remote update
git checkout -b 2.6.38 2.6.38.stable/master
will add a 2.6.38 branch tracking that stable repo. git takes care of the rest.