They've worked together commentating since so I'm sure there would be no problem having them on the show together. Also, as people have mentioned, they seem to get on well again now - even if deep down there is still a bit of hurt there from Karen.
If it is Bryan and Carmen, which I'm sure it must be unless they are referring to the number one couple since they retired, it's great to see them on the show. They are such legends.
I would love to see Michael and Joanna (the number one couple since Bryan and Carmen retired) on one week though, the crowd would go wild for them, as they always do exciting choreography. I would also love to see Slavik and Elena (ranked number one with the weird points system, but really the number 2 couple), but not sure if they would do it.