Two positions have some benefits..
if you know how to tether your bet to a perceived advantage.
Consider:
1)Cuts down on the number of players, speeds up play, prevents some jumping in and out and there is more room for each player.
2)You have more chances for double-downs,splits and naturals.
3)You get faster results.
4)You play twice as many hands per time spent at the table.
5)Your results are more meaningful statistically, because of increased play.
6)It increases your options(playing decisions).
7)Increases your chances of one of the positions being on a winning streak.
On other side of the coin, besides what Dog Hand mentioned, one position:
1)Is easier to control the betting.
2)One position can accomodate a smaller bankroll, even at a high limit table.
3)Is less likely to be wiped out in one shoe.