This used to be possible with all networks in the early 2000's, but with the advance of Bluetooth in car devices, it was stopped..
AFAIK it had nothing to do with Bluetooth devices, it was the software being used by the networks that couldn't handle two IMSI numbers in two cell positions at the same time.
Orange still offer this but only on business plans.
Orange offer as part of their "one-number" deal so you can ring one number and multiple phones have the ability to answer it. Vodafone also offer this as their "multi-sim" deal,
With regard to cloning the sim, i assume its posssible if you have the correct equipment.
If you have a pre 2001 Sim it's fairly easy, as all you need to clone the sim are the IMSI and KI numbers
The IMSI is usually written on the side of the sim, so fairly easy to get, it's the KI number that causes the problem. This can only be read electronically, pre 2001 you could ping the sim and it would reply with it's number, newer sims don't do that and information is encrypted. You can attempt to force it to reveal the KI, but the sim will lock and "die" after about eight attempts, and as there are several thousand possible combinations the chances of you hitting on it are better than winning the lottery, but not by much
ask someone like STARLIGHT in Birkenhead market. If anyone can do it they will be able to.