I don,t know about siberian iris because this will be my first year with any( I got them last fall) but with other iris each rhizome makes a bloom so thats how they are divided you seperate the rhizome and give each room to multiply. The large rhizome in the center probly won,t bloom again so when the clump becomes crowded that one is usually removed to make more room for the rhysomes it produced.

Siberians often take a year or two (or three) to reestablish and start blooming after being divided. They will live, that isn't in question, but don't expect much from them this year.