I'm presuming one of them is your wife's car because as far as I was aware you are only entitled to one tax free car per year (and bike, and boat, and caravan).
Anyway things may have changed but a few years ago a friend of mine had just bought a brand new car and within 3 weeks she received a posting order back to the UK. She didn't have to pay the tax back because at the time she bought the car she was unaware of said posting. She was, however, unable to sell it until the usual 12 month period of ownership was up or she would have faced the usual...banned from buying tax free car for x number of months, pay the tax on it if sold before the 6 month point of ownership etc etc.
I've no idea what paperwork she filled in but if you contact BFG VLO I'm sure they'll be able to give you a definitive answer there and then rather than read a load of opinions on here from guys whose only knowledge of BFG vehicle rules & regs is the current price of fuel coupons!!!