Odds tend to be published for playing one spot against the house. But it is clearly more advantageous on favorable counts to play two spots. It reduces variance. So much so that the optimal bet (total) is 150% over the one spot bet (75% on each spot). This is a fact from application of Kelly's criterion.
Where, if anywhere, is there any data on win rates playing in this way? For example, in Atlantic City, upping bets to 8 units (while it won't get you barred) leads to a great deal of variance, since some of those bets will be losses. But playing 5 units on two spots, or so, seems a little more tolerable....and is better play.
The odds can't simply be calculated from taking a probability table for a one unit bet, given a strategy, and randomizing, because there is a covariance between the two spots (given that you face the same count and cards coming up on both spots).
Wong, in Professional Blackjack, discusses the optimal betting and covariance, but not the resultant win rate from varying your bets between one and two spots, if you can. I've got to think this helps the player quite a bit.