In order for you to receive a call, the GSM network must know your location. Your phone logs into the the network as soon as it is powered up (except 'flight mode'). If you had two phones - they would be both be 'tracked'. Only one call can be answered in one location without creating problems but it would suite your purpose since you would only want to use one.It's called SIM cloning. But it's not allowed on any wireless carrier I know of and outlawed in most countries.
It makes it hard for law enforcement to pin point your location And rumour has it that they use this technology to eavesdrop on ppl too.
The problem is that GSM SIM cards after 2004 are extremely difficult to break (duplicate). That is why the both the NSA & British Intelligence have been accused of trying to infiltrate one of the leading SIM card manufacturers to get SIMs produced that they can hack.