2 Different Oracle DB Version on same server Is it possible to install and run 2 different oracle version(11g and 12c)software and db on same server?
is it possible then how we can? and if not then why?  

Thanks 

Rahul Joshi
Yes, it is possible.

https://docs.oracle.com/database/121/NTDBI/intro.htm#NTDBI2648


  Oracle Database supports multiple Oracle homes. You can install this
  release or previous releases of the software more than once on the
  same system, in different Oracle home directories. This allows
  flexibility in deployment and maintenance of the database software.
  For example, it enables you to run different versions of the database
  simultaneously on the same system...