I noticed on the Vogon Wish List under Video (from some time ago?)
Secondary video window for DosBox monochrome support. Some games also had the option of outputing monochrome to another monitor for debugging purposes.
I interpret this as two SDL windows on a VGA monitor one emulating CGA/EGA/VGA and the other emulating MDA/Hercules monochrome.
Is this implemented, planned, not likely ?