it isn't.
+1
No-one much cares these days.
However, if the OP wants to be ultra-traditional, I was taught at school that the correct format for a married couple is:
Mr & Mrs <man's christian name> <surname>
If he has an honorific:
Dr & Mrs <man's christian name> <surname>
If they both have honorifics:
Dr <man's full name> & Dr <woman's full name>
If they're an unmarried couple, I believe you follow the rule for honorifics, but place the woman's name first, but I don't think I was ever actually taught that at school (Unmarried couples? Shocking! Not for tender young children to know about!)
Tbh, if I were the OP the traditional style is the one thing I'd definitely avoid. Year after year, the only time anyone in my family ever seems to even notice the address on the envelope is to roll their eyes in annoyance at the obnoxiousness of "Mrs <his name>". That complaint is now a tradition, and Christmas wouldn't be the same without it. But it can lead to a less prestigious position in the card display.