Is it possible: MySQL semisync and async replication on same master-slave pair?
Semisync Replication will switch to async on the fly if needed.

I mentioned this before in my old post MySQL Replication : 1 Slave / Multiple Masters

In that post I said


  Having this setup gives you block-level replication within both data centers. If you are using MySQL 5.5, you can have Semisynchronous Replication send SQL to the other DataCenter without waiting for the SQL to be executed, only acknowledged. This keeps any intermittency that standard MySQL Asynchronous Replication would normally cause a MySQL Master to a minimum. In the event of any sudden network latency, MySQL Replication between DataCenters would switch from Semisync to Async. Once conditions improve, then MySQL Replication would it switch back to Semisynch.


In another old post of mine, With MySQL Replication, what level of resilience is possible?, I mentioned the option that shows that semisynch can switch to asynch and back:


  rpl_semi_sync_master_wait_no_slave : With semisynchronous replication, for each transaction, the master waits until timeout for acknowledgment of receipt from some semisynchronous slave. If no response occurs during this period, the master reverts to normal replication. This variable controls whether the master waits for the timeout to expire before reverting to normal replication even if the slave count drops to zero during the timeout period.


Therefore, you cannot setup semisync and async replication on the same instance of MySQL. Then again, you don't need to since it is designed to toggle between them depending on response time or lack of response time.

BTW to setup Semisync Replication, the semisync plugin activated on Master and Slave. See my post Is MySQL Replication Affected by a High-Latency Interconnect? on how to install Semisync Replication.