MAP-Process Unstructured SS Request is the MAP Message used for USSD Processing.
in HLR IMSI Related to MSISDN
in MSC/VLR IMSI related to MSRN
IMSI is used as Common Reference between HLR and MSC For Call routing.Obviously MSC Provides MSRN Per IMSI Based Query.
And this is Temprory until that call.
Mapping TWO IMSI in VLR-Not Possible.This is out of GSM STandards and I never seen such a thing.But Mapping can be possible in HLR.