Dual main characters

When I started the Mason & Penfield Mysteries I had this notion that there were 2 people, John Mason and Ron Penfield, who would share protagonist duties.  But I wanted to avoid a certain number of relationship types that had been done before:

• Mentor/protégé — This has been done a lot. (If you’re is tempted to say mentee rather than protégé, be warned: I have snipers hiding in the ceiling where you’re reading this.  Capiche?)  Mason, the police detective, has a protégé, but he is (mostly) used as Mason’s extension.
• Movie/drinking/sports fan/etc buddies — This relationship is too casual to make sense for a murder mystery series.
• Strangers who meet and combine forces — I find this more plausible than the recreational buddies above; it offers plenty of opportunity for tension between the dual (duelling?) MCs.  But every conflict would be a surprise to them as well as to the reader. I wanted more familiarity between them.
• Partners — Again, there is plenty of room for conflict, but without some hook (like the detectives on the TV show Common Law), it’s difficult to make them unique.
• A/B partners — i.e., one partner starts the investigation, then turns it over to the other.  While this provides structure, there’s no reason for the 2 people to not be 1, since they function as 1 anyway.
• Husband/wife — think Tommy & Tuppence. Don’t think Hart To Hart.

What I settled on was 2 guys who were former co-workers, and who had a grudge between them, a grudge caused by one’s misbehavior; who had gone down separate career paths in the meantime. In Unthinkable, Mason is forced to investigate Penfield; by the end, each understands the other a little better. A few months later, in Seen Sean?, their friendship is strained by Penfield’s actions, but they wind up cooperating to reveal the killer.

My WIP (work in progress), [title redacted] is starting out a little differently: We begin with a dead body and lots of police procedure.  This puts Mason center stage, and at my current point (about 20% of the first draft is complete), Penfield is just stepping onto the stage.

Two questions:

• What do you think about the interplay between Mason and Penfield?
• What other dual protagonist types can you think of?