This is why, notwithstanding the not insignificant money invested in that PCB already, I've personally come to the conclusion that using a second RPi with network master/slave configuration is the way to go. Of course that then opens up three or four (or more!) screens and each gets an hdmi interface and its own GPU too.
I do agree that a dual(+) screen RDP facility is required functionality but there are lots of hurdles to making a VGA or HDMI interface for the RPi, which in any already has all of that built-in and likely costs less money. It just needs software
