Absolutely. In fact, When they jumped up, Jack whispered "come on, come on" and grabbed Ennis and took his hand and put it on himself again... you can't see it completely but it's quite obvious when you look at that scene, where his arm is going. And this time Ennis didn't pull back his hand. And then Jack took off his coat, and only then Ennis mumbled "what are you doing?" So yeah, he knew exactly what Jack was doing. But something else that occured to me, as far as Ennis knew, maybe Jack just wanted to jerk off together, bring each other off. He didn't tell Ennis what he wanted him to do - it was Ennis who chose to go all the way and have a full sexual intercourse.