How do I join two .rar folders?

[Octavian Florea]

A rar file (not folder) is an archive - it stores and compresses content.
The rars you have may be a split archive (if their name is 
something.rar.part1, something.rar.part2), in which case all you have to 
do is to extract the first one, as the second will be extracted 
automatically. If it is not a split archive, then you will have to 
extract each of them in turn. Either way, when done, you will get all 
the content from both rars.