How to use the NVIDIA driver with a PCIe x1 710 and a AMD video card in the first PCIe x16 slot

Install the NVIDIA driver, "sudo mhwd -i pci video-nvidia".
Then edit /etc/mkinitcpio.conf, remove "nouveau" from the MODULES line.
Then run "sudo mkinitcpio -p linux48", change 48 to the version of your kernel, 44 is the default that comes with Manjaro.
Look in the /etc/X11/mhwd.d/nouveau.conf file, you need the BusID line.
Add BusID "PCI:5:0:0" or whatever your BusID line is, to the /etc/X11/mhwd.d/nvidia.conf. It goes in the Device section, under Driver "nvidia".

Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    BusID       "PCI:5:0:0"
    VendorName     "NVIDIA Corporation"
	Option "NoLogo" "1"
EndSection

Then reboot, and it should work.
This assumes you already blacklisted the amdgpu driver.