but for a different reason. I wanted a second SIM so that I could operate two handsets on the same number (don't ask).
I know someone who works at a fairly senior level for one of the networks, and I tried to get a favour, but she told me the same story - you can't have a second SIM on a personal phone account under any circumstances. She said I could have extra SIMs if I entered into a commercial contract, and had more than eight different numbers, but otherwise no deal.
Certain cars are more likely to be driven by business users, and maybe that's why they have the facility.