This shouldnt be a problem for you. SQL 2005 clusters very easily. Like MarkusB said, make sure you have second failover group and the virtual-instance name is different and you give it a different IP address.

I had a gotcha with MSDTC once so if you use it make sure it has its own drive (not sure why but that is the only way we managed to get ours working. In its own group too!).