They won't cross since the two alliances could have different goals. If they did join up, it would be more like the supernovas against the Yonkous and the NW. None of the captains of the supernovas are thinking of joining up with all of their generation just to battle it out with the supernovas. Its more of they each have different goals, but we know that Luffy and Kidd seem to have similar goals. And as such, would not go well to be allied together. We still don't know what is the goal of the other alliance either. For all we know, the other alliance could be up against Shanks as their goal. If it were Shanks, then Luffy might leave Kidd's alliance to go and then handle business with the other supernovas to stop them from going after Shanks.

Actually, after saying that, it would probably be a viable option after dressrosa and/during country of wano. It is totally like a pirate to go take on/down other pirates that could be heading to take on your own target.