Don't know how this is handled with prosumer dervices but BES servers do sync with the PIN (kind of an address of the device). You would have to tell the BES each time the PIN of the other device. Don't think this is possible.
If you're using a BIS account, this may not work. If you are not using the SK65 on the carrier that supports it (ie any north american carrier) then you may not be able to put the PIN and IMEI in the BIS account because it's not on their allow tables.
I say might, but I could definitely be wrong.
If it does allow you to, and you are using a BIS account, all you need to do is swap SIM card and plug the PIN/IMEI into the BIS site.
If you use a BES, swap SIM cards and activate the new handheld on the BES.
If you can only do OTA activations, then you'll need a new EA password each time, but the SK65 can't do Enterprise Activations. Wired activations only.
I use 2 BB's. 8700c and 7100g. When I switch the sim I log in to my bis acct. Change the pin and IMEI. Then I usually shoot the new device service books and internet service books. I also register the new device on the network through the HRT (Host Routing Table) option. I haven't had any probs.