Was working on this issue for a few days while updating my desktop (High Sierra, GTX 950). Booting with two monitors connected (one DP, one DVI) caused a black screen and hang. Booting with the DVI cable disconnected works fine, and after booting I could re-connect the DVI to get the second monitor working, though it would black screen/hang again if the computer went into sleep and turned off the displays.
Also, it seemed that WhateverGreen.kext was not successfully loading, and I could not find it via IORegistryExplorer.
Re-enabling CSM support in my BIOS is what ultimately fixed it for me (The desktop guides I read all suggested disabling this during installation). After re-enabling it, I can boot with dual displays, and WhateverGreen is showing up in IORegistryExplorer.