SLI with 2X Nvidia Geforce GTX 690 on Ubuntu 20.04 not working...

I've just realised that only one of my two Nvidia GEforce GTX 690s is actually working with Linux. The Nvidia xserver app recognises both cards but as far as Ubuntu 20.04 is concerned there is just one card. This possibly accounts for why Skyrim has never run quite so well on Linux as it did on Windows. xconfig is installed but the SLI command is not recognised. I can only set SLI to 'off' and 'mosaic'; not to 'on'. I've tried everything within my very limited powers. Nvidia suggest I have the 'wrong' kernel so maybe that's next on the list. But before I do something drastic regarding Ubuntu (which I like very much), I'd really like some opinions. SLI seems to have a bit of a mixed reputation and some Linux people seem to think it is 'broken' on Linux.If I'm wasting my time trying to get it to work then I'd like to know; and why, if possible. If I should give up and get a new card I'd listen to that too so recommendations as to which (I play Skyrim and take photos and I don't do much else on a computer these days) won't be received with huffiness... Hardware isn't something I know very much about at all (I've replaced graphics cards though) and my Linux knowledge is rudimentary... I don't want a new computer yet as I'm hoping that Bethesda will release TESVI in my lifetime so would rather wait and spec it to fit the game...

Thanks in advance for any help!