You can do a discard
merge.
This has to be done from the command line. Open up the Developer
command
prompt,
then navigate to a folder under either of your branches (i.e. navigate
to one of the affected
workspaces).
Then type:
tf merge /r /discard "$/Project/B1" "$/Project/B2" /v:C12345~C12345
This will take the changeset identified (in this case it was changeset
#12345), and update it as merged to the target branch (branch B2). The target files will be checked out, but they will not be changed - you
can simply check them in to complete the operation. After that the
changeset will no longer appear as a merge candidate. You can specify
a range of changesets to merge at the same time, but they should be
contiguous.
Note that after doing this a changeset will occasionally still show up
as a merge candidate - this is rather uncommon with the latest
versions of TFS, and it is virtually impossible to fix (unless you are
running your own local install of TFS and want to get your hands very
dirty in the database). If you end up with one of these marooned
changesets, just ignore it.
Source:
Finding merge candidates in TFS
Note: When the command has finished, you still need to check in the merge.
For more tutorials, you could also refer below blogs: