They won't get merged or overwritten as the name of the PC is stored in the path.
The structure of file history is like this:
<Drive>:\FileHistory\<User name>\<Computer name>\Data\<Drive letter>\Users\<User name>\etc etc
So you'll end up with paths like this on your FileHistory drive.
You can check this for the PC you are already backing up with file history if you open up the FileHistory folder on the backup drive.