In SD with 70% pen with 2 players and the dealer
you only get 4 hands per deck... is this better say
7 hands in DD (70% pen, 3 players and dealer.) ???
A rule of six single-deck game with two players won't get you 70% penetration. Depending on how you judge penetration, at best it is around 62% penetration. Depending on the rules, the two-decker might be the better choice, since you are getting nearly 75% penetration.