As others have indicated, SpectraLayers is a complex beast, and I certainly don’t pretend to be an expert on it, especially when it comes to any complex editing.
My sense is that some of the others who’ve responded have some good ideas on at least trying to work the lead/rhythm guitars scenario, whether or not SpectraLayers comes into play and to what degree if so.
The Unmix Song thing (i.e. which does the first level of stem separation in SpectraLayers Pro) is really pretty straightforward, and the Unmix Instrument, which lets you “train” (to a degree) SpectraLayers to separate out an instrument that isn’t covered by Unmix Song could be something to try if the lead guitar and rhythm guitar sounds are sufficiently different (perhaps it could help if the rhythm guitars were acoustic and you had a distorted lead guitar???) – probably operating on the guitars stem, rather than the full mix (if it will be helpful at all for this task). You can find basic videos for both of those on the page with SpectraLayers 12 new features:
The feature that can (possibly) separate lead and backing vocals is called Unmix Chorus. Once you’ve separated the vocals from the rest of the stems, you run the Unmix Chorus on the vocals layer. Here’s a video that demonstrates that (and it actually works pretty well in the video, unlike in my attempts – the link goes straight to that section):
As for learning more about SpectraLayers in general, Steinberg has a page with some tutorial-type videos at:
I’ve also gone through some courses on Groove3.com (I have their all-access pass, and their videos are often useful for coming up to speed with music-related applications, so, in my book, worth the something like $99 a year for the all their video and non-video courses plus access to lots of online Hal Leonard sheet music-type books). The courses that turn up in relation to SpectraLayers (not all of which are relevant) are at:
https://www.groove3.com/search?filter=&keyword=spectralayers
The SpectraLayers Explained and SpectraLayers 12 Update Explained courses would be especially relevant, and the Spectralayers 11 Update Explained would be the one that would be most likely to go into detail on the Unmix Chorus.
Good luck.