sql cluster multiple instance

Can someone explain to me how SQL 2000 with multiple instance (active/active) clustering on Windows 2003 works?

Also, is there any way

I have an active/passive SQL cluster with a SAN right now and understand that. From what I’ve read it sounds like multiple nodes do not access the same resource simoultaneously but rather they have a local copy of a seperate instance. I must be mis-understanding this because it just doesn’t make sense.