3 monitors on 2 GT 1030 GPUs?

I have two GeForce GT 1030 Silent LP 2GB (gv-n1030sl-2gl) GPUs, and have been trying for weeks to get them to power 3 monitors on my AsRock Taichi X370 MoBo. My OS is Linux Mint (Cinnamon) 19.1 (Tessa), and I have been working both with nvidia-settings (v. 390.116) and xrandr (v. 1.5). I have gone back to earlier nvidia-settings versions as well, and have also tried working on this problem within Mint 18.3. Both GPUs are detected in nvidia-settings, although I cannot assign a single Xscreen across the GPUs. (I presume that this is because the GPUs are not “SLI Ready”.) Only one GPU is detected by xrandr (even after running “xrandr --setprovideroutputsource 1 0”). As best I can tell, Base Mosaic requires a single Xscreen. And according to everything I’ve read online, Base Mosaic is necessary for a multiple GPU setup with multiple monitors. So my question is, “Am I correct in concluding that it is impossible for me to get a 3-monitor setup running on Linux with these two GPUs?” And, if yes, “Is this because they are not SLI Ready?”

Did you already try just to use the BaseMosaic option? What error was in the logs?
A different approach would be to use a DVI splitter-cable if the connector is a dual-link one.

Thanks for the response, generix. If I am understanding you correctly, then it is possible for me to get my two GT 1030 GPUs powering 3 monitors even though they are not SLI Ready. Could this be true? If so, then I’m happy to work with you on getting the GT 1030s working. Would it be helpful for me to send a current /etc/X11/xorg.conf and a corresponding /var/log/xorg.0.log? Also, wouldn’t a DVI splitter just give me two identical screens? If so, that is a non-starter, because I want the screens extended and not to mirror one-another. (To be honest, I am not sure what a dual-link setup would look like in my case.) As for using BaseMosaic, that was not an option under advanced options in nvidia-settings’ GUI (version 390.116). Differently put, the GUI did not allow me to choose the same Xscreen for monitors in different GPUs. In previous versions of nvidia-settings, a check-box for the BaseMosaic option would appear but did not work (in other ways that I can no longer recall).
nvidia-bug-report.log.gz (577 KB)

Please run nvidia-bug-report.sh as root and attach the resulting .gz file to your post. Hovering the mouse over an existing post of yours will reveal a paperclip icon.
https://devtalk.nvidia.com/default/topic/1043347/announcements/attaching-files-to-forum-topics-posts/

Okay, generix. Here’s the .gz file. I’ve posted it to the previous post as well.
nvidia-bug-report.log.gz (577 KB)

The bug report is of a configuration that actually recognizes both of my GT1030 GPUs. Getting the configuration required using Xinerama and an Xscreen for each monitor. It is the best I could obtain by modifying xorg.conf using the nvidia-settings GUI (v. 390.77, buildd@lcy01-amd64-022). (As I mentioned previously, the GUI does not provide Base Mosaic as an advanced option.) The configuration’s shortcomings are that my desktop’s background stretches among all monitors (as opposed to the usual repetition of the background for each) and–much more importantly–that icons on the desktop cannot be moved. If try dragging then dropping an icon, the icon does not move (on first drag-drop attempt) then all icons disappear (on second attempt). Not good.

Xinerama has its downsides and issues, you also still have compositing enabled, that’s probably why you can’t move the icons.
Let’s try a basemosaic setup:
Please disable iommu in bios, also set the kernel parameter

iommu=off

to make sure.
Then use only this as /etc/X11/xorg.conf:

Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce GT 1030"
    BusID          "PCI:45:0:0"
    Option         "BaseMosaic" "true"
    Option         "AllowEmptyInitialConfiguration" "true"
EndSection

Please create a new nvidia-bug-report.log after config changes and reboot.

Now I’ve lost the screen on my 2nd GPU. I’m attaching the latest bug report. Again, thanks for your help, generix.
nvidia-bug-report.log.gz (582 KB)

BaseMosaic doesn’t work:

[    10.020] (EE) NVIDIA(GPU-0): Failed to find a valid Base Mosaic configuration.
[    10.020] (EE) NVIDIA(GPU-0): Invalid Base Mosaic configuration 1 of 1:
[    10.020] (EE) NVIDIA(GPU-0): GPUs:
[    10.020] (EE) NVIDIA(GPU-0):     1) NVIDIA GPU at PCI:45:0:0
[    10.020] (EE) NVIDIA(GPU-0):     2) NVIDIA GPU at PCI:46:0:0
[    10.020] (EE) NVIDIA(GPU-0): Errors:
[    10.020] (EE) NVIDIA(GPU-0):     - The video link was not detected

With Geforce type cards, it’s always a hit or miss.
So you’re left to a Xinerama setup. for that to work a bit better, you’ll have to turn off compositing which limits you to only be able to use non-compositing desktops like xfce or kde.
To disable compositing, add

Section "Extensions"
    Option "Composite" "Disable"
EndSection

to your previous Xinerama-enabled xorg.conf.

So I guess that the answer to my first question is, “No”. To my understanding, Cinnamon is a descendant of gnome, which is inherently non-compositing. And since I’m more attached to Cinnamon than to the two GT 1030s, I need to switch to another GPU (in my case, one that can drive all three of my monitors). So how in the first place could I have kept from making the mistake of buying 2 GPUs that could not jointly support Base Mosaic? When shopping for GPUs, one can always determine whether or not they are SLI Ready. Is this the criterion that I should have been looking for? Or is there some other determining factor? Again, many thanks for your help on this, generix.

Any Nvidia gpu beginning with Kepler (GTX6xx/Quadro K) can drive up to 4 monitors if it has enough connectors. Newer displayport links can be used to drive two (1080p) monitors either by a hub or daisy-chaining. E.g. a single 1050Ti has three connectors, a Quadro P620 has four.
BaseMosaic is special. It’s Nvidia proprietary, mainly meant as selling point for Quadro type cards, with those it’s working up to 16/32 monitors. With Geforce type cards, this is artificially limited to 3 monitors and often just doesn’t work. No really reliable way to tell beforehands, like said, hit-and-miss.
As a rule-of-thumb, if you want to drive up to 4 monitors buy a single card with enough connectors. If you want to drive more monitors, use Quadros and BaseMosaic. Of course, you’ll also have to take into account the resolution of the monitors regarding having enough video memory.

I’ve got it: Buyer beware if you want to install more than one Geforce GPU in your computer!

Exactly. When it comes to graphics, multi-gpu is almost always a bad idea, especially on consumer hardware.