DoubleDOS was able to run two DOS programs side by side and hotkey switch between them. They would DoubleDOS Procomm, and then DoubleDOS a command prompt in the other one.

I had a friend who ran the Hellfire Club BBS, and he decided to DoubleDOS the WWIV BBS in one session, and then DoubleDOS Test Drive in the other. When he switched to the WWIV BBS, he had garbage in that screen. Apparently it didn't handle direct to hardware memory writes too well or something.