We have several projects that use a common submodule, albeit often on different branches.

(1) Current versions of SourceTree will not open multiple instances of a submodule if the name is the same (Sourcetree 1.9 and before had no problem with this).

(2) More egregiously, if you have one instance open, and request to open a second (separate) instance of the submodule (associated with a different project/repo, and possibly on a different branch), SourceTree simply takes you to the first open instance! This opens up all sorts of problems with the user thinking they are working on one branch of the submodule, when in fact they are modifying another branch.