In TortoiseHg for Windows it can freely close one or the other or both heads of the same branch. It is important, however, to update to the head you wish to close before issuing the commit. This is what is issued by the UI:
% hg commit --verbose "--message=Close default branch" --close-branch
As an advice, and because it is a better practice, you should use named branches instead of committing multiple heads on the same branch. In fact, pushing multiple heads of the same branch is not recommended, and usually requires a --force option.
In the end, I am sure SourceTree will be more cooperative with named branches anyway...
A good read on the topic can be found here.