IMEI's have nothing to do with Sim cloning.

The IMEI is written in the device, in a sim clone it is the IMSI that gets copied. For an "official" multisim they simply provide you with 2 sims with the same IMSI, usually with A and B stamped on the back. Other than that the sims are identical. I've never heard of an operator having to "authorize" IMEI's, the nice thing about GSM is that you can use as many IMEI's as you want, all the operator cares about is the IMSI which it has stored away safely in its systems. The sim sends a challenge-response style temporary version of the IMSI, but in the case of 2 sims it is still based on the same number.

The IMSI is what keeps you safe, if someone got a hold of the sim's IMSI they could make calls on your account. Which is why they used a complicated key exchange system to verify and protect the code, it is this system that got hacked in the late 90's (COMP128 V1) and then protected again in V2 of that protection method. It has since then not been hacked, which is why it is not possible to clone current generation sim cards.