I have read that in order to get the New Frontier silver strikes set of all 4, you need to play both machines. I personally found this to be NOT TRUE.
I played ONE machine, located closest to the gaming tables, and for $30 play I was lucky enough to get a full set plus one extra skull-pistol strike (5 total). I thought it was one of the more loose macines I've played, but we all get lucky once in a while. This happened to be last Monday (4-30-01) about 2:00PM.