Usage patterns
相方を務めるto serve as someone's (comedy) partner
He serves as the partner of a popular manzai comedian.
〈人〉を相方に選ぶto choose someone as one's (comedy) partner
He chose his childhood friend as his partner and started doing manzai.
相方とコンビを組むto form a duo with one's partner
The two of them form a duo with the partner they've had since high school.