OK, I’m probably the only person who has ever had this happen, but it is confusing. We have two monitors setup going to two computers. One has DVI connected to both and the other has HDMI connected to both. This allows each computer to drive two monitors or when both are up, you just switch from DVI to HDMI to get the one you want.
Windows 8.1 seems to handle this pretty well. You can set a primary display and the logon prompt appears there. There is another screen and the computer is sending output but you only look at it when you want.
However with Ubuntu and nVidia, this doesn’t quite work. Even if you set a screen as primary with
nvidia-settings, this is not honored on boot. What happens is that only one monitor is chosen. This is in hardware the one attached to the first DVI port, so what you see on the other screen is a Ubuntu logo, then the sound and then it goes to a logo without the logon screen. Pretty mysterious, but if you have these dual monitor configurations, make sure to plug it in and when it boost properly, that is what is happening 🙂