In Odoo 9 POS, There is no any functionality to manage multiple stores.  But yes you can use similar feature Multi Company Setup instead of Multiple stores.

Let me Explain How:

(1) Setup one Main Company and Multiple Child Companies.

(2) Create Multiple Session with Multiple Users in POS.

(3) In User Setting Add Allowed Companies as Main Company as well as User Related Child Company.

(4) Also Set Current Company as Child Company for this User.

After this Steps you can use POS with Multi Session with Multiple Users and Multiple Stores(Child Companies)

Hope this solution will help you.

Thanks!