From what I read on Star Wars Wiki, it sounds like he choosed to spare Jacen to save Ben from the Dark Side. Also it gives me the impression that Luke was on top most of the time...
Btw love that queto.
"I don't feel any Jedi presences at all, and if I don't, than neither do you-nothing real, anyway."
"That's because he doesn't want you to feel him, but he's here."
"And I suppose you'll help me find him if I let you go? Nice try."
"I don't think you'll need any help finding him, Jacen. Dad's right behind you."