Some Software features and procedure:

1) Software makes an "Image" of the SIM card in the internal socket. It stores the internal network ID (telephone number, contract cellular plan and operator's ID) and contacts too if you want it in the memory of the phone.

2) Then, you decide if to use the "phisical Sim" or "Virtual" SIM. If you decide use the Virtual SIM, you can put out the phisical SIM and store it in a safe place. Anyway you are paying the contract bill. Or you can still use the phisical one.

3) Once done, we can turn off the phone and put in the socket another SIM and repeat the operation as many times as we want.

Now, with the "Images" of our SIM in the phone the software can mount it in "virtual sockets" so we can use all the SIM's images simultaneosly or the one we choose.

So I think... if cheap phones can do it well with 2 SIM, why no our Nokia N900?