My blackjack hand calculator shows that with six decks and dealer stands on soft 17, player 6+5 vs. dealer ace, the following expected values of each play:
Hit = +0.147596
Double = +0.129710
Stand = -0.661883
Hitting, being limited to one card, would be like doubling, but without doubling the bet. The expected value would thus be half that of doubling, or +0.129710/2 = +0.064855. That is less than the expected value of doubling of +0.129710. Therefore, if you're limited to one card after hitting, then you should double.