After spending a lot of time dicking around with xrandr, I found a solution that works for me.  I'm using this forum as a means to provide others the solution to my problem, as well as to ask my own questions about certain things I don't understand.Setup:2x Nvidia GT 630 (each with 2x DVI 1x HDMI)    3x Monitors (2x Acer1680x1050 (DVI) & 1x LG  1680x1050 (DVI)) Debian 8.6 default install(GnomeDesktop Manager 3) Nouveau drivers (from default install)I have 1 Acer and 1 LG attached to Card 1.  Default install has them functioning perfectly as a seamless desktop.  Last Acer on Card 2 is not detected in GDM and is non-functional.  It IS detected deeper in the system, using some BASH magic I grabbed from someone who knows what they're doing:ls /sys/class/drm/*/status | xargs -I {} -i bash -c echo -n {}: ; cat {}/sys/class/drm/card0-DVI-I-1/status:connected/sys/class/drm/card0-DVI-I-2/status:connected/sys/class/drm/card0-HDMI-A-1/status:disconnected/sys/class/drm/card1-DVI-I-3/status:connected/sys/class/drm/card1-DVI-I-4/status:disconnected/sys/class/drm/card1-HDMI-A-2/status:disconnectedI start by typingxrandr --listprovidersand getProviders: number : 2Provider 0: id: 0x64 cap: 0x7, Source Output, Sink Output, Source Offload crtcs: 2 outputs: 3 associated providers: 1 name:nouveauProvider 1: id: 0x36a cap: 0x2, Sink Output crtcs: 2 outputs: 3 associated providers: 1 name:modesettingRight away you can see that Provider 0 has it's name entry set to nouveau and Provider 1 name entry set to modesettingI'm quite sure each provider is its own discrete graphics card.  Why is one set to use nouveau and the other modesetting (presumably KernelModesetting)?  Nouveau is supposed to run above KMS...Dmesg shows me that both cards are running with nouveau...Typing xrandr --setprovideroutputsource nouveau nouveauappears to force both name entries on both providers to nouveau.  X immediately reboots (or crashes) and I'm brought back to the login screen.After logging in, in an X terminal I type:  xrandr --listprovidersand get confirmation:Providers: number : 2Provider 0: id: 0xbf cap: 0x7, Source Output, Sink Output, Source Offload crtcs: 2 outputs: 3 associated providers: 0 name:nouveauProvider 1: id: 0x64 cap: 0x7, Source Output, Sink Output, Source Offload crtcs: 2 outputs: 3 associated providers: 0 name:nouveauBoth cap entries are the same.  Why is this? What does cap mean?From here I can actually enable xrandr to talk to the second card (Provider 1):xrandr --setprovideroutputsource 1 0Now the extra DVI and HDMI ports will show up when I type:xrandrFrom here I enable the 3rd monitor with:xrandr --output DVI-I-1-3 auto(Because my 3rd monitor resides on DVI-I-1-3)I can instead choose to do enable the monitor via the GDM GUI app under settings.From here some more xrandr output positioning commands or GUI manipulation will get my monitor where it should be relative to the others, and everything works.  Now I have to wrap all these commands up in a script because xrandr does not save settings on reboot.  I'll discuss the script in a later post when I get it working.Some problems currently are poor graphical performance when moving windows around and sometimes the mouse freezes for a couple seconds at a time.  The proprietary Nvidia drivers might fix this, but other solutions are most welcome.I can't help but think that I've gone about this wrong and that there's a better way to achieve what I've just done while still using xrandr (I've also tried a custom xorg.conf using ZaphodHeads without results).  Why did I have to type:xrandr --setprovideroutputsource nouveau nouveauTo setup the rest of the commands in the first place?Xrandr helpShows:--setprovideroutputsource  <prov-xid> <source-xid>Where can I find a list of source-xid's?Why are the DVI ports labeled differently between xrandr and /sys/class/drm/ ?
Multi-Graphics Cards, Multi-Heads, 3+ Monitor Display using Nouveau, GDM3 & Debian 8.6
debian;xrandr;multi monitor