When there are two protagonists, one male and one female, they are usually love interests.
When there are two protagonists, both female, they usually have separate plot lines with little interaction.
When there are two protagonists, both male....
What is the typical relationship there? And is this really as uncommon as my ignorance would suggest?
EDIT: To clarify, I'm talking about books told through two POVs, where both characters have a claim to being the protagonist. I'm interested in how central the relationship is to the story, because with male-female it's usually very central vs female-female, rather peripheral.