Two GPUs. Can't get a single Xscreen

Hi. I have been working on this for weeks and could not advance any more. I will really appreciate the help (pulling my hair out here!)
I have 2 GeForce cards, a GT710 and a 210. Driving 6 monitors. 4 on the 710. Using last driver that supports them both (right?) 340.108.

Display app does not see the two monitors on the 210 and Nvidia-settings shows them as disabled and orange and only gives the option of putting them in a new Xscreen. I have done a lot of things things in this forum and others, anything I can but have ended up having to reinstall everything a couple of times. I am giving up doing it on my own. I really need some step by step guidance here, don’t want to mess it all up again. I’ll send all info requested quickly. Thank you.

(Everything works in windows without issue).

inxi -Fxxx

  Host: krafty Kernel: 5.4.0-131-generic x86_64 bits: 64 compiler: gcc 
  v: 9.4.0 Desktop: Cinnamon 4.6.7 info: mate-panel wm: Metacity 3.36.1 
  dm: LightDM 1.30.0 Distro: Linux Mint 20 Ulyana base: Ubuntu 20.04 focal 
  Type: Desktop System: Gigabyte product: N/A v: N/A 
  serial: <superuser/root required> Chassis: type: 3 
  serial: <superuser/root required> 
  Mobo: Gigabyte model: 970A-DS3P serial: <superuser/root required> 
  BIOS: American Megatrends v: FD date: 02/26/2016 
  Topology: 6-Core model: AMD Phenom II X6 1055T bits: 64 type: MCP 
  arch: K10 L2 cache: 3072 KiB 
  flags: lm nx pae sse sse2 sse3 sse4a svm bogomips: 33757 
  Speed: 804 MHz min/max: 800/2800 MHz boost: enabled Core speeds (MHz): 
  1: 804 2: 804 3: 804 4: 804 5: 804 6: 804 
  Device-1: NVIDIA GK208B [GeForce GT 710] vendor: ASUSTeK driver: nvidia 
  v: 340.108 bus ID: 01:00.0 chip ID: 10de:128b 
  Device-2: NVIDIA GT218 [GeForce 210] vendor: driver: nvidia 
  v: 340.108 bus ID: 06:00.0 chip ID: 10de:0a65 
  Display: x11 server: X.Org 1.20.13 driver: fbdev,nouveau 
  unloaded: modesetting,vesa resolution: 1680x1050~77Hz 
  OpenGL: renderer: N/A v: N/A direct render: N/A 
  Device-1: NVIDIA GK208 HDMI/DP Audio vendor: ASUSTeK driver: snd_hda_intel 
  v: kernel bus ID: 01:00.1 chip ID: 10de:0e0f 
  Device-2: NVIDIA High Definition Audio vendor: 
  driver: snd_hda_intel v: kernel bus ID: 06:00.1 chip ID: 10de:0be3 
  Sound Server: ALSA v: k5.4.0-131-generic 
  Device-1: Realtek RTL8111/8168/8411 PCI Express Gigabit Ethernet 
  vendor: Gigabyte driver: r8169 v: kernel port: d000 bus ID: 04:00.0 
  chip ID: 10ec:8168 
  IF: enp4s0 state: up speed: 100 Mbps duplex: full mac: e0:d5:5e:34:84:2d 
  Local Storage: total: 6.15 TiB used: 33.09 GiB (0.5%) 
  ID-1: /dev/sda vendor: Crucial model: CT256MX100SSD1 size: 238.47 GiB 
  speed: 6.0 Gb/s serial: 14200C244FCD rev: MU01 scheme: MBR 
  ID-2: /dev/sdb vendor: Seagate model: ST4000DM004-2CV104 size: 3.64 TiB 
  speed: 6.0 Gb/s rotation: 5425 rpm serial: ZFN35P1G rev: 0001 scheme: GPT 
  ID-3: /dev/sdc vendor: Western Digital model: WD5000AAKS-00V1A0 
  size: 465.76 GiB speed: 3.0 Gb/s serial: WD-WCAWF2048522 rev: 1D05 
  scheme: MBR 
  ID-4: /dev/sdd vendor: Western Digital model: WD20EARS-00MVWB0 
  size: 1.82 TiB speed: 3.0 Gb/s serial: WD-WMAZA3562623 rev: AB51 
  scheme: MBR 
  Hardware-1: VIA VT6421 IDE/SATA Controller driver: sata_via v: 2.6 
  port: c000 bus ID: 05:06.0 chip ID: 1106.3249 rev: 50 
  ID-1: / size: 111.83 GiB used: 16.05 GiB (14.3%) fs: ext4 dev: /dev/sda5 
  System Temperatures: cpu: 20.5 C mobo: N/A 
  Fan Speeds (RPM): N/A 
  Processes: 259 Uptime: 1h 26m Memory: 23.46 GiB used: 3.41 GiB (14.5%) 
  Init: systemd v: 245 runlevel: 5 Compilers: gcc: 9.4.0 alt: 9 Shell: bash 
  v: 5.0.17 running in: gnome-terminal inxi: 3.0.38 

Rather forget about that, at least with the proprietary nvidia driver. The 210 is simply too old, the 340 driver out of support and dead and that feature was only added in driver 465 and up.
With that combo, you could only either use the nvidia driver in conjunction with Xinerama (which is a pain) or nouveau.

Can’t tell you how thankful I am to receive an answer! Now I don’t have to struggle uselessly… I’m low on money and wanted to make it work. Now I just want to ask you if taking out the 210 and buying another 710 or a 730 would make things work easily with the 470 driver? (Automatic single xscreen for all monitors). And are there any other limitations I might run into? I would like to be sure before purchasing. Thank your very much for your kind attention.

With gpus supported by driver 470, this is a no-brainer
Limitations are, the primary graphics card has to have enough memory for applications and output buffers, i.e. the 2GB of the 730 would be enough for about 6 FHD screens (rule of thumb: 350MB per FHD monitor).
Personally, I can’t recommend getting a Kepler based graphics card now, though. The 470 driver only has 2 years of nvidia support left now (distro patching probably longer), so its death is foreseeable.
My conclusion: If you can get a 710/730 real cheap, consider that but check out prices for a T400 (adapter cables might be needed, no VGA) and compare.

Afterthought: don’t you have a cpu with integrated graphics that you can enable to achieve the same without a second nvidia graphics?