Is it okay to have two oracle clients (both 11g and 12c) in the same db server? What are the potential issues I could face? The reason im doing this is to connect the app server where I'm deploying the clients to two databases each running on 11g and another on 12c.
You should not install more than one Oracle Client (per architecture) on one machine. For sure they do not work in parallel.
By heart I know several issues:
You would have to change
PATH settings in order to determine which Client you want to use.
TNS_ADMIN environment variable is used by both versions and must be set accordingly. Oracle OleDB provider can be installed only once (per architecture). Version of Oracle Data Provider for .NET (ODP.NET) must match the client version. Using ODBC can be also a challenge.