We have done this, partly as the "blood relative" will always have a special bond with their niece and nephew, so nominating them as godparents is unneccessary (to me). Their partner is also the uncle/aunt, but naming them godparent gives them a particular, special role in that child's life.
We don't take godparents to mean raising our child if we were to die, but a moral counsellor whose responsibility it is to take an interest in the child and be their mentor if the child is going through difficult times (ie. divorce, they would step in to make sure the child had someone to talk to outside the parents, failing exams, struggling with life ambitions etc).
My DH is my niece's godfather, and I was delighted he was asked to be her mentor etc, I thought of it reflecting well not only on him, but me too, as I brought him into the family etc.
I'm sure no offense was intended - we nominate 2 males & 1 female for boys and 2 females, 1 male for girls - so sometimes not possible to ask the couple together, as the numbers don't exactly work - in fact we never asked couples as godparents, always one or other.