We witnessed over the last few days the one real reason that you might want to run two clients, even on a single processor machine. The servers went down, several times. When that happens you cannot turn in work to the downed server, but you can sometimes get work from another server and the fact that it slows down both clients will help to keep you systems folding for a longer period of time while you wait for stanford to fix the servers.
How?? You only need to copy the fah.exe, charmm.prm, opls.prm and place them in another folder with a different name. Once there create a shortcut for the fah.exe and add the '-local' switch to the shortcut so that that it will instruct the client to run in that directory only. I have included a pic so that you can see what I mean. Then start the client with the shortcut. You will have to enter your information and it will get everything else it needs from the servers.