Often when you play blackjack you’ll see someone playing two spots instead of one. Is there any advantage to this? Let’s see.
Most of the time that players switch from one spot to two (or more) they do so because of the misconception that by “changing the flow of cards” they will presumably change the luck of the table. Unfortunately this is not so. You are just as likely to keep on losing, as you are to start winning when you switch to two spots. There is no mathematical basis for this and it is definitely not one of the reasons you would want to play two spots.

One obvious thing that happens when you play more than one spot is that you will be dealt more hands per hour. Gaming expert Alan Krigman developed this nice rule of thumb for determining how many decisions each spot will average per hour. If N is the number of positions in action, then each player will be dealt 420/(N+1) hands per hour.
For example suppose you play heads up with the dealer. You can expect to have 210 decisions per hour (420/2). If you decide to play 2 spots, then each spot will get 140 hands per hour (420/3). That means you’ll be getting 280 decisions per hour on your two spots compared to 210 hands per hour by playing only one spot. That’s an increase of 25%.

Getting more decisions per hour is not necessarily good especially when the casino has the edge. Say you are a basic strategy player and your average bet is $10 per hand. Suppose you are playing with three other players. Using the above rule you can expect to average 105 decisions per hour (420/4). Wagering $10 per decision means you will bet on average a total $1050 over the course of an hour. If the casino has a 0.5% edge, then your expected hourly loss is about $5. By playing two spots instead of one, the number of decisions you will get per hour on each spot is 84 (420/5). This amounts to a total of 168 decisions per hour for the two hands. At $10 bet per decision, you will average $1680 worth of bets with an expected loss of $8 per hour. Therefore betting the same amount on two spots when the casino has the edge compared to betting half as much on one spot will increase your hourly rate of loss.

However, there is one benefit of playing two spots compared to one. You will experience less fluctuation in your bankroll. Your greatest fluctuation occurs when you bet $20 on one hand and the least fluctuation occurs when you bet $10 on two hands. The reason for this is that sometimes one hand will win and the other will lose. Therefore if you want to decrease the swings in your bankroll, you can do so by betting two spots instead of one. But one caveat. By decreasing your fluctuation (or variance as mathematicians like to call it) you will not win as much when Lady Luck smiles on you (likewise you will not lose as much when she is frowning on you).

The biggest advantage for playing two spots occurs when you are card counting. If you know you have the edge on the next hand, it’s to your advantage to play more than one spot. If you are playing heads up with the dealer, by playing two spots you will have twice as much chance of getting the high value cards as the dealer (because you are playing two spots instead of one). Also, instead of betting say a top bet of $100 on one hand, you’ll have the same risk by betting 75% of that amount on two hands (i.e. bet $75 on each of two hands). This technique also allows counters to get more money on the table when they have the edge on the next hand without drawing to much attention from the pit bosses (having $100 bet on one hand looks more serious than $50 bet on two hands).

I generally play two hands when I count, not only when I have a big edge, but also sometimes when the count is neutral or barely positive. By jumping from one hand to two hands early on it creates the perception that I am a gambler. I usually increase my bets on one of the hands if it wins and the count is still neutral or slightly positive while keeping my bet size the same on the other hand if it lost. Of course I’m still betting a relatively small amount even on two hands compared to when I have a very positive advantage. And when this occurs I try not to bet the same amounts on each spot. For example I rarely bet $50 on two spots. Instead I may bet $60 or $70 on one spot and $40 or $30 on the other. Again this creates the perception that I am a gambler rather than a skillful card counter.
Card counters will also bet more than one spot if they have the edge and it looks like the next hand will be the last one before the shuffle. Here it’s to your advantage to bet as many hands as you think you can get away on that last round.

Another technique used by the pros is to play as many as three playing spots with small bets when the count is negative. This accomplishes two things. First it consumes the “small” cards so the deck will go positive and the counter can then bet up. Secondly, if a pit boss suspects the player is counting and observes him spreading to three hands, he may have second thoughts that he is card counting.
The bottom line is that playing more than one spot can be a bad move if the casinos have the edge over you, but a smart move if you have the edge over the casinos.