Think about your closest friends. If you know them well enough, you also know their strengths and weaknesses. You know how they think, how they react, how they tick. You trust them to know the same about you, too. You might even trust them enough to let them fix your car — but not drive it. Hell would freeze over before you let them drive it.
In action-oriented stories, two characters who are friends will work with each other during combat. Often this will present the image of Back-to-Back Badasses. If one person is big and the other short, the big guy will use brute strength and the short guy will use finesse. Maybe the big guy will clasp hands with the short guy and fling him around as though he was a weapon. Or the big guy will use the short guy as a projectile.
No matter what, they will fight in tandem. They will watch each other's backs. There is something genuinely satisfying about watching the heroes take out a room full of bad guys by working together.
Bash Brothers has been suggested to play 6 roles. Click below to see other actors suggested for each role, and vote for who you think would play the role best.