With Feeder Sync there should be no issue with enabling FM2 on your production database while leveraging replication to maintain your FM1 Responder geodatabase.
You will need to ensure that Feeder Sync is enabled when editing with FM2 in order to maintain the legacy FM1 field information in your production Geodatabase.
Let us know if you have any other questions.
There is no issue withe 'mixing' FM1 and 2, as Responder is Feeder Manager version agnostic. However, it should be noted that mixing 10.1.1 and 10.2.1b, while there are no KNOWN issues, is an unsupported and untested configuration.