hehe,
I don't think it is serious.
Once I turned in my old I94 card to the officer when I was entering US.
If you enter Canada from US, if you Canada is your final desitination, and you are not staying there for more than 30 days, usually the boarder control officer (by driving) or the airline worker (flying) won't take your I94 card. You can use them to come back (I think).
I guess you guys can try to come back with I94s. If there are problems then use AP, and try to turn in the old I94 to the officers at the boarder (they are usually nice..)
Even if you keep your old I94 cards, it is not a big deal. I had the same problem and asked my attorney, and he told me no one cares about the old I94 cards, the most recent one is the valid one.