I tried to do this (adding SQLName\Instance2 to existing SQLName\Instance1) today (SQL 2008 SP3 cluster) and confirmed it would NOT work @@

I was hoping it would work so I can consolidate multiple old SQL cluster instance names onto new clusters using DNS Alias to re-direct the SQLName part

e.g.

SQL1\I1

SQL2\I2

>> merge into >> SQLNew\I1, and SQLNew\I2 (so a 2-to-1, or N-to-1 merge)

guess the only way to have multiple named instances is a stand-alone server

SQLName\Instance1

SQLName\Instance2

SQLName\Instance3 ...