The problem is it'd need all the dual-link pairs, which HDMI doesn't have. What I had in mind was taking the TDMS pairs for RGB from HDMI2 and combining with HDMI1, using its clock and whatever else is necessary. A hacked up Y cable, basically. A cunning driver would be able to present data in a format the monitor can eat - it seems that it sends every second pixel on a different link which is a bit tricky to mangle. Would be nicer if it was half and half - for side by side display on the one screen! I'm guessing it would only be practical with some trickery that exists as support on the SoC specifically for dual-link DVI.
One advantage is that all the 'old' 30" monitors of this type have that one mode, so one hack would do all. I gather it can be converted using an active HDMI to displayport adapter, then displayport to dual link DVI, but cost of each of those devices probably exceeds an RPi.
I've used an Rpi2 for years as a thin client driving a 1920*1200*60 screen no problem (well, the odd green flash). It's only 16 bit colour, but anything useful over RDP (anything but video) seems to dither colours nicely. I so badly wanted dual screen, and now that seems to be a reality (could never get true dual screen with VGA666). But greed has kicked in, it'd be nice to run these big monitors off something small and modern, rather than a full-size PC (even a SFF PC is not easy to get dual link DVI cards for).