the MSC knows the new cell, thanks to the message “HO PERFORMED”, which is sent from BSC to MSC after the handover is succesfull finished.
regarding the handling of a second call, i don’t know.. it should be explained in 3GPP. In my opinion, the paging is not needed : the MSC will send one message on FACCH (or SACCH) such as “Call waiting”, or something.
In this situation, and in my opinion, there is no need for paging, authentication, ciphering, call setup, alerting, …