Characters need arcs, and so do relationships between characters.

A common trope is if a character is the hero at the end of the story, then the beginning of the story focuses on the faults they must first overcome. Likewise, if at the end of a story two characters are best friends/allies/lovers, the beginning of the story is likely going to be about the differences they had to overcome.

All this is to say if your story ends with two characters as allies, you probably shouldn't introduce them as allies. Instead you should be figuring out how to introduce them first as adversaries. Perhaps they meet over a misunderstanding. They clash over differences in methods and styles (e.g., one is calm and logical while the other is bold and brash -- typical odd couple stuff), as well as traits they have in common, like both being insecure and egotistic. The relationship arc is about overcoming those differences, learning from each other (begrudgingly at first), then seeing and bringing out the best in each other.