Ben Skywalker and Galen Marek would make a better match up, though. Tahiri is a loser, so that would be boring. If we're counting her as Jacen's apprentice, then just drop both of them, in accordance with the "no outside powers" concept.
If we're having Caedus using Ben Skywalker as an apprentice, then in all honesty I'm not sure whose side either apprentice would be on.
For starters Ben and Caedus are both Vader's grandkids (he is more apt to figure out Ben is his grandson than Caedus). Second Caedus killed Ben's mother, and took a few potshots at Luke as well.
I really don't see Vader letting Galen bump off Ben, I mean look at how Vader kept trying to capture Luke instead of killing him.
So the first thing that's likely to happen is Ben would defect to Vader's side or decide to just strike out on his own... In the former Vader would have two "apprentices" competing over the who gets to be apprentice, while Vader goes to bump off Caedus.
Sorry but I don't think Jacen could take on Darth Vader, Galen Marek, and Ben Skywalker at the same time. If you're going to give Caedus an apprentice, at least give him one that wouldn't jump ship or try to run him through at the first opportunity...