There was a Concurrent DOS, and there were quite a few others: topview, taskview, doubledos, vm/386, to name a few. I remember reading this article (p. 22) sparking my imagination as a kid: http://bitsavers.trailing-edge.com/magazines/Microsystems_Jo...
For a long time I figured that was just the right way to do multitasking. Hook the interrupt and pass control to the next task when you are done. I also figured most of what I wanted was popup TSRs anyway - how can a human possibly pay attention to more than one program at a time? Makes me wonder how much my brain has changed over the years by being exposed to a multitasking and multiple windows on a large screen.
People who just wanted mutitasking DOS had DesqView and ... "concurrent"? was it? i forget.
I wrote TSR's that snatched cycles off the timer interrupt and rode 21h for opportunity to flush buffers to/from disk.