Is it possible to setup multiple RW Replicas for a database in an AlwaysOn Availability Group? Can I have multiple RW replicas of an AlwaysOn Availability Group database across different servers? (I.e. two-way replication so that multiple servers can write to the same DB.)
No, it is not possible to have multiple read/write replicas in an Availability Group. For your requirement, you might consider Merge Replication which allows writes at multiple databases and merges the data across all databases involved in the replication.

This doesn't allow for writing to that same DB, but the replication architecture works to keep both databases synced.