Technically, you could even do dual monitors with IBM PC, by having both a CGA card and a MDA/Hercules card. Obviously, the utility was limited, but apparently people did it.
For graphics yes. But around that time all Borland (C, Pascal) products supported debugging with to monitors (VGA+Hercules). And I used a little TSR for years that transferred the contents of the main VGA Screen to Hercules via 'PtrScr' key, which was handy too. All text mode of course.
That said, I literally was born after Windows 3.0. So... I never personally dealt with such a setup. Maybe some day, as a matter of exploring older computers, I will get a chance to actually run such a setup and see what it was like.
It's probably worth mentioning that the idea of developing Windows software in Windows was something that only came around until 1990-91 or so. Prior to that, all the tooling (except for things like the message Spy and the resource editor) ran in the command line in text mode, including the debugger.
Visual Basic gets a huge amount of grief, but when it was first released, this was a huge part of the reason why it was so amazing. It took Windows development from a relatively complex and obscure process involving a multi-step build/edit/compile and turned it into 'drag the button where you want it, click, and add an event handler... all in Windows itself.'
A lot of graphical software had commandline switches to override the videocard autodetection, "in case it failed", but this could also be used to force the software onto one monitor or another.
I ran dual-head under DesQview for quite a while. The CGA monitor whined like mad so I kept it off most of the time, but when I needed more display real estate, I'd turn it on and put my text sessions over there, leaving the graphical stuff on the VGA.
But OP is amazing to do this btw.