Part One: Cad Bane and Jango's rivalry
We now know it is canon that Bane and Jango were rivals, and that they knew each other. This isn't a lot to go by, granted, but I think we can get a lot more out of Bane and Jango's relationship based on a few things: Jango's career, Bane's career, Boba's career, and Bane and Boba's relationship.
First of all, there is the fact that we have seen Bane training Boba. Unfortunately it is still unclear as to how or why Bane began training him in the first place. Their new "master/apprentice" relationship could have been initiated by Bane because he felt an obligation to help out young Boba (I'll elaborate more on this theory in a bit). It could have been Boba hiring Bane to train him. Or they could have simply taken a job together and the training part is a secondary element to their partnership. The reasons as to why Bane ended up training Boba has not been confirmed yet, so until then we have to speculate.
But suppose Bane did decide to go out of his way to train Boba. Could this have been because he personally knew Jango?
If we go by this theory - that Bane is training Boba because he knew Jango - than obviously that means Bane and Jango had at least some sort of a companionship. We know they were rivals, but that does necessarily make them enemies. They could have been close rivals in the bounty hunting profession, but outside of their work they could have been "frenemies", or partners - perhaps more than that. Even if they did not always get along (and I imagine they clashed often), one thing is clear: their relationship was close enough that Bane would train Jango's son after his death. If Bane despised Jango (or visa-versa), there is no way that Bane would end up training Boba, or even working with him. I cannot see Bane wanting to have anything to do the relatives of someone he hated. So, since Bane is training Boba, that means Bane and Jango got along to some degree. Maybe they saw their rivalry as more of a friendly competition.
In fact, since Bane rarely works with others and prefers to work alone, I would also add that Bane and Jango must have been fairly close - or at least, had some sort of connection in their past, which would have been strong enough that they would honor it even while becoming rivals. It is already canon that both Bane and Jango value some level of respect and honor. We see this with Jango's personal moral code, and we see it when Bane saved Hardeen, and in both of them in how they respect the Jedi. Jango's sense of honor is a lot stronger than Bane's, but that's beside the point. Also, we know that Jango cared a lot about the people closest to him, and he worked hard to give Boba the best training possible.
But if Jango really did love Boba so much and wanted to train him to be the best, this brings up a small problem. Jango was killed before Boba could finish his training. And furthermore, Jango already lives a very dangerous lifestyle. So wouldn't he have known there would be a risk that he couldn't finish Boba's training? Wouldn't he have considered the strong probability that he might be killed while Boba was still young?
Considering that Jango is a very efficient, well-prepared sort of bounty hunter, I believe that yes, he did put those probabilities into consideration. And I think this means that Jango had a back-up plan for Boba's training in the happenstance that Jango was killed. So what sort of a plan would Jango have made? What if, before he was killed, Jango found some way to make sure that Boba's training would be completed if Jango could not finish it? What if Jango found someone he could trust to train Boba if this happened? If so, what sort of person do you think Jango would hire? I think Jango would have hired someone he knew was very skilled, very experienced, and could be trusted to train Boba. Someone with a strong enough sense of honor that they would respect this one request of Jango's. This someone could very well have been Cad Bane.
This is my "Passing the Baton" Theory: Bane is training Boba to honor Jango's request that Bane complete Boba's training upon Jango's untimely death.
I have the theory that Jango sent a personal request to Bane, asking that he would finish Boba's training if Jango died before he could finish it. And Bane respected Jango enough that, after Jango was killed, he eventually honored this request. Of course, there is quite a timespan between Jango's death and the Bounty Hunter arc (which I can only assume is set around two years into the Clone Wars). Just because Bane would honor his last request, doesn't mean that he would do it immediately. Bane is a busy guy, and wouldn't drop everything as soon as he heard what happened to Jango.
Plus, there is the fact that Bane became the most feared hunter in the galaxy because Jango was killed. That ended their rivalry, and Bane quickly rose to the top and became the best. Bane obviously likes being the best, and he enjoys basking in the glory. I can totally see him taking his time in honoring Jango's request to train Boba. He enjoys being in the spotlight, takes a bunch of jobs now that he is the most feared hunter, and gets around to training Boba when he is in the mood for it. Like I already mentioned, Jango's sense of honor is much stronger than Bane's. Bane would probably justify being lazy in honoring Jango's request as long as he ended up honoring it eventually. Or he only decides to honor Jango's request when it is convenient for him.
In order for this theory to work, Bane and Jango would have had to trust each other fairly well. And there also would need to be a reason that Bane would want to honor Jango's request. I have another theory for why Bane may respect Jango that much, but I'll go more into that in Part Two.
You may remember that in Season Two, Aurra Sing was temporarily training Boba. However I don't think this works against my "Passing the baton" theory. In that situation, Boba was seeking revenge for Jango's death and wanted to kill Mace Windu. He probably needed immediate assistance, or he didn't want to wait for Bane to help him. Or, Aurra could have stepped in before Bane had the chance to help Boba out. Also, based on what little we have of the Bounty Hunter arc in which Bane was training Boba, there didn't seem to be any reluctance in Boba to be trained. If we do end up seeing this arc in comic or book form and Boba starts pulling the "I don't need you to train me" attitude, the "Passing the baton" theory still works because Boba may not have wanted Bane to finish his training, even if it was his father's last request. Boba may still be reeling from Aurra's betrayal of him, and that could cause tension between Bane and Boba.
In summary, I think that there would have been a lot of tension between Bane and Jango, but not enough to make them despise each other. Furthermore, they had to have some sort of connection that would make them close enough so there was at least some trust between them. Boba probably wouldn't want to be trained by someone his father hated, and Bane wouldn't want to train the son of someone he hated. And if Bane is training Boba because Jango asked him to, it makes their relationship even more interesting.
Sadly, it was mentioned at Celebration that this arc would have gone into all of Bane and Jango's backstory, so until this story is released in a book, comic, or other, we don't know for sure how they got along aside from the fact that they were rivals. But it's still fun to think about. In Part Two I will go more into Bane and Jango's similarities, and how this might have more to do with their relationship, as well as Bane and Boba's relationship.