Merging from branch line to branch line was introduced in 3.1.0. The merge form was a little buggy there, though, so I would recommend switching to 3.2.x instead.
How exactly is it not possible?
When I have my branch open and select Merge changes here... and select Advanced Merge, I can select a branch and revisions to merge.
Do you mean you get an error? I do believe the merge is/was broken in 3.1.1.