You can even add multiple entries in the [paths] section of your .hg/hgrc file.
[paths]
default = /repo_store/hg/project1
sandbox = /repo_store/hg/project1_experimental
And then can specify its alias in the mercurial commands. default repo need not be specified but others have to be like,
hg in # check incoming changes from default repo
hg in default # check incoming changes from default repo
hg in sandbox # check incoming changes from sandbox repo
hg pull sandbox # pull changes from sandbox repo