I'm curious with this myself. Ive done it both with skiving right up to the stitch with the stitch still being in full thickness leather. This seems to work well, except doesn’t like to spread evenly, or run through the double needle guide well. When ive skived the edge to sew in the skived area, its easier to fold and lay over, but im nervous of the strength of the seam, and the thin material doesnt feed through my machine well. It will have a tendency to bunch up, particularly in the start and stop when back tagging. I've never skived double the salvage width. I just assume it will be equally difficult to stop bunching, and i dont think when folded over that the two skived layers will equal the thickness of the unskived leather, leaving a ridge away from my seam width of my salvage.

Well I had a bit of a play today as I had to get on with the dashboards and came to the same conclusions as Lee but I hammered the seam down before top stitching.However when I did the dashboards I didn't bother skiving as the way the seam sits the extra thickness isn't visible.I'll post a pic when I've finished. I do have other stuff coming up that I will need to skive the edges on though.

i would suggest you lower your sew allowance, if you try and skive an edge at 10mm on auto leather across a dash its going to be hard work, your not going to get a even skive in my opinion,
you should be sewing in the skive so it folds clean (flat) with minimal salvage imo

So your saying skive at about 8mm to allow for 4mm on each side of the fold and give enough room for a top stitch or 2mm skive, 1mm each side and top stitch only one layer of leather.I can see what you mean by an even skive, I just see what the eastern europeans on instagram can do and try to get closer to it, they seem to run their skiving machines at a really slow speed and get such a neat even cut.I need to have a play and a bit more adjustment though.
Pic of completed dash.

Just a small tip from the side...you can skive diagonal or as a step...depends which look you prefer...if your margin is 8mm then skive 10mm (you need to fold the leather) and then make a top stitch...and skive slowly to be accurate...the more you put attention to details in every step the better the result will be...it s not up to how fast can you do something, it up to how clean can you do something...