The 500 already has a thinner gasket from factory than the 600 or 700 so the compression is higher already. If you do split run very good fuel all time or it will ping and cause issues.
Just use 2 layers. That should put it somewhere in the neighborhood of < .8mm. Spray both sides of warmed h/g w/ copper coat to ensure that you won't have to take it back apart for a leak.
I seem to remember that 97 & up twin heads had less squish than the 94-96s. Those two layers plus copper coat should be perfect. I definitely wouldn't go thinner than the 2 layers you mentioned earlier.