Here it is:

AlwaysOn Architecture Guide: Building a High Availability and Disaster Recovery Solution by Using Failover Cluster Instances and Availability Groups

http://msdn.microsoft.com/en-us/library/jj215886

Here is the excerpt I am talking about:

"...

Instance Naming and File Path

The two FCIs must use different instance names within the same WSFC, for example, using “INST_A” as the instance name for the primary FCI and “INST_B” as the instance name for the DR FCI. (In contrast to availability groups, database mirroring permits each FCI to use the same instance name if the FCIs are on separate WSFCs. In Figure 1, both FCIs used the same the same instance name, INST_A, with the FCI+DBM solution).

..."