How to setup SQL active/active cluster to achieve Blue / Green instance switching?
What you are looking to do is basically have two instances running that you can redirect clients between.  There's no need to do this within Failover clustering as this won't help you any.  Doing this simply requires that you setup two standalone instances of SQL Server, each on a separate Windows server.  Then setup a CNAME in DNS and use that to point people to the correct SQL Instance.

Now for some of the problems.


The data won't be in sync between the two instances.
DNS takes time to replicate and update on the client machine's local cache, so you'll end up with users connected to both systems.


A better solution would be to take a snapshot of the database before doing the schema change release.  If the release is successful delete the snapshot.  If the release fails and you have to rollback you'll need to restore the database from the snapshot.  This will be pretty quick, but there will be an outage while the rollback happens.

What you are looking for basically won't work.