Select non display connected GPU and custom non standard resolution

[Iggyboo]

Just stumbled upon this and am amazed at how great it is. Been using Steam Remote and Parsec on my old classic Acer Aspire laptop with an 18.4in screen (good luck finding a laptop now days with that) but with a Core2 Duo 2ghz processor. However, it's a bit 'non standard' on the native resolution: 1680x945. I've added that resolution in the settings page but still shows 1600x900 which is what the connected monitor is on the system. Incidentally, I have a second GPU in this system which is the same model of the other. That's two AMD RX580 which I experimented with in crossfire but gave up as it's just not supported for remote play which is the main purpose for this system.

v {TLDR Question here) v
I'm wondering if I could use that 'unused' GPU as the "host" for Sunshine with a custom resolution just for connecting via Moonlight. I don't see a way in the settings to make it use it since it's the same name in the dxgi-info tool. Would there be a hidden identifier I need to set?

Also I see when it starts up it shows "Capture size : 1600x900" and "Virtual Desktop : 1600x900". Are there any settings I can force to make it use a different size VirtualDesktp/CaptureSize to my native on laptop 1680x945? Perhaps it doesn't even matter as it does still look pretty good.

Thanks!

Edit: Forgot, I'm using Window10 on the host system