In TFS, I normally right-click a file and select Merge to merge it. What if I have two files I want to select to include in a single merge? How do I do that?
Interesting. I just tried it - although you can multi-select items in Source Control Explorer, the context menu then disables Merge.
You can, however, right-click a folder and select Merge.
It looks like if you have two files among many in a folder you would have to merge them individually (or possibly use the command-line TF tool).
If the two file you're trying to merge have an existing branch/merge relationship, you can right click the source file -> Merge and choose the desired target file from the dropdown.
If not, and you just want a simple 2-way diff, right click one of the files -> Compare, then enter the name & version of the file to diff against. At this point it's up to whatever client diff tool you've configured to do something constructive with the two files sans common ancestor.