They're both really good defenders, in a team that generally has a huge proportion of possession. In most matches it doesn't particularly matter that they don't seem to fully trust each other. They don't face a whole lot of attacks, and their quality is enough to handle a big proportion of what comes their way.
It's only when it comes to the crunch, against the top teams, that it becomes an issue.
I didn't like seeing them arguing on the pitch in the middle of the match today. And I can't help but get the impression they each find the other difficult to work with.
Their personal relationship doesn't really matter...until it does.