The two devices obviously have different SIM ID's, so the question is whether they can both be associated to the same phone number. The answer is that you should talk to your service provider. A quick search revealed AT&T in the States allowing something similar to this, but all of the hits I got referred to that same carrier so I don't know how widespread such a service is. It will certainly be a decision for the service provider though, as they associate SIM numbers to phone numbers, rather than a feature of the technology or anything the user can control, so they are the only people who can answer this.