Yup.....I took 2, cout'em TWO, hammerheads in that 500 pt list.
Really, people will be able to cheese out no matter what you do. I think the reason to have only one troop choice is fairly simple....it let's you have more fun. A lot of codexes will be fairly limited in what they will be able to do in 500 pts if forced to take two Troops choices. IN the one 500 pt game we did I think that was kinda the point, and it was fun, but in a doubles it should be much more about the matchup.