Shouldn't this be in Marvin?

Anywho - running a modern operating system (like Windows 7) you probably aren't going to accomplish this; no DOS mode, no monochrome, etc. I'm not sure something as old as DOS 6.22 or what-have-you would have proper drivers for a PCIe card, chipset, etc on the other hand. My advice would be to go with older hardware that originally was able to do this kind of thing, as that capability should not have changed with age.

If you're debugging something modern you can probably just run the entire thing within a VM or window the application in your IDE and enable step-through or JIT. No need for a specific debug display, but an additional monitor may be useful (and most modern cards will provide that by themselves).

Main problem is PC architecture. There cant be 2 VGA cards at once. They confilct in memory area - both loads vga bios into same area. So 1st you to need to find PCI card, that can be disabled for dual screen mode (think that matrox call it slave mode). These cards were made for multimonitor systems - each card got one monitor. They have switches, with them you can disable vga bios and you can have more than one vga card then.
For example Intergraph G91 (http://www.vgamuseum.info/index.php/vlasks-co … itek-power-9100) or Matrox Millennium (http://www.vgamuseum.info/index.php/vlasks-co … -2064w-is-storm) have such switch and support this. 2ND problem is how to enable that card in dos. This setup was intended for windows system, not sure about dos. Maybe someone did dos driver, which can enable 2nd card, but im not sure about this (you have to search dos utilities - best will be checking matrox 1st).
Probably easiest will be using vga/ega combo cards (they can be set by switches even to ega/MDA modes), but these are only ISA 8/16bit cards. Havent seen any PCI card made like that.