Sorry, I was mistaken in my previous post.
Now after extensively testing, I can confirm here that running with -video bgfx -numscreens 2 causes an indefinite hang.
Running with -video d3d -numscreens 2 I get Unable to create the Direct3D device <88760868>
Using opengl everything works as it should.
I'll have to disconnect everything here and reseat all the connections to see if it's a specific fault here on my end.
Interesting, I used to get the exact same error code if I tried to Alt+Tab from full screen MAME in D3D mode (or was it Alt+Enter that caused that? I forget), it used to require a force kill via Task Manager (or a Ctrl+C in the command line) as the error dialog would repeat infinitely. It didn't happen in OpenGL.