Hi , can i write two select queries in my single query database service ?...

Two select queries can't be used in query database node. Reason for this was explained under Activity Execution in the below link
forum.appian.com/.../Query_Database_Smart_Service

You can try using query rule to achieve it in a single script task node.

Try mapping the Table to a CDT via a data store and use query rules to pull the data out.