Joe and Nicky both entered the jedi world very young. When they were kids, the small age gap between them was enough for them not to know each other much.

As they get older, Joe becomes Andy's padawan and Nicky becomes Meta's padawan. Under Meta Kozak strict teaching, Nicky is taught to regard Andromache's wilder stance with the rules with disdain, and pretty soon Joe and Nicky are as close to ennemies as two jedis can be. (so technically on the same side, but really not liking each other). Thankfully, they have very few interactions.

Problem is, Meta is actually a sith. Now, she knows Nicky won't turn to the dark side, but he does trust her and won't question her when she gives her a mission. So under false pretense, she uses Nicky and his great talent to do something bad (idk, steal something, cause a catastrophy...). When it's over, Meta is long gone, and Nicky is crushed, deeply broken, letting himself arrested by other jedi and be judged by the council.

It is quite obvious that Nicky was manipulated, and didn't know about Meta. So, while Nicky is old enough to be on his own, technically, they decide to put him with another master to hopefully get him to find peace again.

Of course, they pick Joe. Joe is NOT happy about the task. Until he is.