Two of my potential PhD referees are married to each other – is that a problem?
I don't expect this to be an issue.

I have been at departments where two spouses were faculty members. I have also received and evaluated applications where there were references for the same candidate from academics who I know are close collaborators and friends. In each case, the natural assumption was that they have their own careers, working style, students, collaborators, and -- of course -- opinions.

In this case, it is clear your exposure to them is in different contexts, and presumably their letters will make that clear. It is perhaps more likely than average that they will talk with each other about their letters as well (given they have more opportunity to do so!), but that happens all the time anyway.