This non working dual monitor issue seems to be happening to quite a few folks, exactly as you describe and not just with an Nvidia card. I have the same thing with Intel 630 and have seen this mentioned in various places and build threads on a couple of forums now. I'm pretty shocked so few people are running dual monitor setups and that this problem hasn't been more widely discussed and even solved for already. Wish I knew more about deciphering drivers and kext files to understand what was happening but I'm about 99% sure this is a sleep issue of some sort. It appears to me that the OS is sleeping certain video outputs, under certain circumstances, during the boot process. For me, the display port goes to sleep if both hdmi and display port are used. I can actually watch my display go to sleep the moment the screen goes blank. I was able to get hot plugging working for a hot minute and then that broke big. Pretty disappointed right now as dual monitors is a huge part of my workflow and has been for years. Once it arives, I'll be trying a 1050 windforce card that I got an excellent deal on but if I can't get that to work, I'm switching to Windows 10, which TBH, is much better suited to high density 4k displays (I can scale the user interface, a far better solution than pixel doubling) and at worst requires a similar amount of fiddling about.
