It's 9,999 USD per person, if you are two, you can multiply that amount for two. I never heard of a "combined limit" of sort - any reference would be appreciated, lucha54!
Actually, we came with slightly more than that and mixing USD and EUR (we were able to sell some stuff last minute and a few euros came in) and nobody checked us.
I was wearing a waistwallet under my pants and carrying my share plus the extra cash, my husband (a citizen) was slightly below the USD 9,999 limit.
I should assume missing EUR and USD mixed up with the dogs' smell?! Anyway, I didn't see no dog, but we were flying from Madrid and perhaps our flight was less interesting than those coming from Miami.