The workaround we use is to make sure that I always get to the car and unlock it before my wife. Once it's picked up one signal, it seems to stay locked to that until the car's locked.
For us it's meant changing the way we leave the house, as I used to be the one that locked the house up, whilst my wife got in the car and got settled in. Now my wife locks up, so she always gets to the car after me. The main downside is that she always faffs around for ages, putting her handbag in exactly the right place, sorting out sunglasses, lip balm, putting on her seatbelt, etc, so now I have to sit there whilst she does all this stuff . . .