Creating headless context for 2nd GPU


I am trying to split work between 2 GPU’s, particularly 2 Quadro k4200’s. I am on nvidia-355 driver on Ubuntu 14.04.

First off, I am trying to SLI them but it doesn’t appear to be working. I’ve installed the SLI bridge and Nvidia settings picks up both GPU’s, but performance numbers seem to be the same. I’ve set SLI setting to SFR in xorg.conf as well. But each time I dump xorg.conf from nvidia-settings as a test, it turns SLI back off. Is SLI not supported for Quadro K4200’s?

Secondly, given the situation above, I also am trying a non-SLI solution of offloading work to a 2nd GPU without having a display attached to it. The work I need done is within OpenGL and to create a context I need to call XOpenDisplay on the display attached to the 2nd GPU. I am able to create a context for each GPU when I have 2 displays connected, one to each GPU. However, I only want to connect one display to the main GPU and have 2nd GPU be processing without any video out. I’ve tried suggestions here:
but each time I try to enter UseDisplayDevice=none and Virtual for the 2nd screen, driver will pop up with the message “The system is running in low-graphics mode” error message. states that UseDisplayDevice=none is supported for Quadro devices. Is there anything I am missing?