Several SIM with Arduino GSM/GPRS shield
You should reset the shield, and keep reset, while switch sims. The reset is on pin 7.

If you look at the schematic, you can see there are only 3 lines, plus a GND and VDD. The GND and VDD you can probably share between the different sims. The SIM_RST, SIM_SCK and SIM_DATA need to be switched.

I think you should use a MUX for that, but that is not my area of expertise. Note that at least the DATA line is bi-directional.