Can use GTX 1080, can't use GTX 660 in dual graphics card setup with 3 monitors

I’ve been trying to setup 3 monitors across 2 GPU’s for a while now, but nothing I do seems to work.
I’m running Debian Stretch with GNOME 3, and I’m using the drivers from the Debian repository (https://packages.debian.org/stretch/nvidia-driver, appears to be driver version 390.116-1? Not 100% sure what that number next to the package means). As stated in the title, my system uses a GTX 1080 and a GTX 660.
When I run lspci, both graphics cards are detected. Similarly, when I use the nvidia-settings utility, it recognizes both GPU’s, and even recognizes the monitor attached to the second GPU. In the nvidia-settings utility, when I click on the “Configuration” option, it only gives me the option to create a “New X Screen”. I’ve tried doing this, but this seems to create a whole new X screen/instance which doesn’t seem to work at all without manually editing the config file. The closest I’ve gotten is to manually go into the generated config file (I named the file 20-nvidia.conf and moved it to /usr/share/X11/xorg.conf.d/) and change the “BaseMosaic” option to “on”. This, if I remember correctly, will let me move my cursor to the other X screen, but the screen is black, the cursor is an “x”, and I can’t actually interact with the screen in any useful way.
I tried using Xrandr, which seemed really promising, but Xrandr seemingly does not detect my GTX 660.
I’ve considered switching to XFCE, as I’ve heard that still supports Xinerama (apparently GNOME no longer does, which is why I want to use Xrandr), but I really don’t want to do the big switch if I don’t have to.

Any ideas?

With that combination, you can only use a Xinerama setup which has downsides (compositing has to be turned off (no GNOME), losing hw acceleration on at least the screen on the 660, if not all, etc.)
On the other hand, the benefit of doing this is zilch. Why don’t you just remove the 660 and connect all monitors to the 1080?

My 3rd monitor is a TV, and it doesn’t have a DP input. (Neither do my other monitors). I suppose I can buy an adapter, but I’d rather not. Why is it that I need Xinerama? Why doesn’t Xrandr work for this?

Two different gpus means you’ll get two separate screens, :0.0 and :0.1, xrandr will only ever work on one of them, it can’t glue them together. Xinerama builds one virtual screen from separate screens. If you also have an integrated intel gpu with a hdmi connector, you can enable that and use PRIME to create one screen with the 1080 but this currently doesn’t really work with two nvidia cards.

I see, I guess I’ll just look for an adapter. Thanks.