Hi Surjit,
Following on Atul's reply, the Installation number is unique across your Landscape i.e. For your R/3 Landscape you must have one Installation number, another from BW, another for SCM and so on.
As you will distribute the current PRD system (SID1) to two PRD systems (SID1, SID2) and both will be still under R/3 Landscape, so there should not be any change in current licensing approach.
1) Moreover, your total number of named users are still same. So no change in the figure.
2) You are going to still use the R/3 (for example) product licenses, you are not going for any new SAP Solution let's say HANA, so you don't have to revise the license procurement.
Let me know what you think?
-Akshay