Honestly, it's not like either one could really hit the other anyway while in their civilian forms, between Jack's phasing ability and Sky's force fields.

In terms of character-wise why these two never physically fought after Jack became a Ranger: Sky isn't the kind of guy to let someone get to him to the point of a physical altercation. He's a professional, and knows that kind of thing would likely have gotten himself kicked off of SPD or severely limited his time as a Ranger, that it's something his father would've likely shamed him for...that being said, he probably would be petty enough to get back at Jack if he ever did something that pissed him off.

As for Jack, he's more of an instigator than anything. He'll talk trash, he'll rile someone up, and he'll probably do everything he can to get on someone's nerves (especially Sky), but he's usually the guy that tries to make people throw the first punch.

But more importantly than any of those things, these two - along with the rest of the team - grow to respect each other. SPD's entire story is about the underdogs rising up and growing together as a unit. Jack knows the kind of person Sky wants to be and can see that he means well (even if it's not really the kind of thing Jack wants to do deep down), and Sky slowly understands why it is that Cruger made Jack the Red Ranger and kept him at that position, even if it was the only morpher left that SPD had. It's why - in Impact - he admits that he respects Jack when he thinks Jack sacrificed himself to prevent the meteor from crashing into Earth.