I think the question is unclear as to the situation. For example: Is the deck "fouled" because;
A. the dealer used the wrong deck?,
B. the dealer mixed cards from the two decks together?,
C. the dealer dealt the hole cards from one deck and the flop from another (If this happened, I really think this person shouldn't be allowed to deal)?
In my opinion, these would be resolved as follows:
For situation A, if the dealer just used the wrong deck, let the hand play out and change decks for the next hand. So what, if the same deck is used for two hands in a row?
For situation B and C, this would be a "fouled" deck and as Nick and K-Lo said, the hand is void, all bets returned and the hand replayed with the correct cards/deck.
Chet