Jeff -
There is no way to specify a named instance with the SET PARTNER = xxx, but you are able to use an alternate TCP port.
Each SQL Server instance should use a different port for database mirroring.
serverA 192.168.1.4:5022
serverA/instance1 192.168.1.4:5023
serverA/instance2 192.168.1.4:5024
Hope that helps!
Mike