Forgot to mention that yes, while I have sometimes bought 2 of something in the past, it is never with new or recent intros - they are expensive enough as it is!
But I have learned (in general, not with irises in particular) the hard way that 1) if I want something, and think I might want another, to get it NOW because it may be gone (never to return) when I make up my mind, and 2) attrition happens (bloomed out, gophers, rot, whatever reason for whatever plant species). For these reasons, getting a second older one can be seen as "insurance".
Newer irises, however, are bound to be around for a while. If ever I need (attrition) or want another one, I'll probably be able to get it for some years after introduction, so only one of a cultivar is bought at a time.