Actually "double team" is not a defensive scheme but a tactical tool anyways. The option of double-teaming an (chosen) opponent´s player once he touches the ball in a scoring position would be an interesting additional choice, I´m just not sure people would be happy with the outcome, as it may lead to plenty "unwanted" side effects and people would run around crying because it´s impelented "in a wrong way".
Double-teaming, just like full court press and 2-3 and such, are actually more or less "surprise" options for short to medium stretches, and I think adding THIS option would really lead to some good results.
So like your base defense will be played for 3 quartes in the game, and you can choose one "special" D which will be used for only one quarter (of your choice, so 1,2,3,4 or OT), and while the opponent coach usually makes adjustment during the game, he cannot adopt to that.
Under those circumstances, Double Team (and Full Court Press) and such make alot more sense.
Zwei Dinge sind unendlich, die Dummheit und das All...