Excuse my language but I fucking hate that.
Mr and Mrs Joe Bloggs... Mr & Mrs Bloggs, fine, I am Mrs Bloggs so that's OK, but Mr and Mrs Joe Bloggs - where's my name gone??
This kind of shit is why I kept my maiden name.
As for the OP, Mrs and Mr has exactly the same amount of syllables as Mr and Mrs, so a PP is correct - it only doesn't sound like it flows, because you don't commonly hear it.
If I was writing a professional letter to two people though, I would primarily address it to whoever I spoke to more.
If I were the primary point of contact for something, and they addressed a letter or email to my husband first, I'd not be impressed.
When I write my Christmas cards, if I'm sending it to a female friend and her husband, I just address the envelope to her.