Seems to me that I have seen a cell with a genuine double anvil, if my memory is correct it was the northern storm on June 10, 2002 in central Nebraska. We were on the southern storm but I remember looking north and seeing two distinct anvils, one right above the other, looked kinda like an anvil sandwich. I remember asking another chaser about it, and being told it was probably the result of two distinct bursts of convection, the first one having produced anvil #1 which sank a bit so that when anvil #2 was produced it was slightly higher up. It's possible I'm confusing that day with another storm but I don't think so... That was my very first trip to the Plains and I asked a lot of questions about a lot of things, the double anvil being one of them. Wish I had a picture, that would clinch it, but I don't. I think this happened relatively early in the storm's life cycle.
Based on that experience I'd have to say that double anvils do happen occasionally, although the one I saw didn't look much like Doug's pic. The main difference being that in my case the anvils were both exactly the same size, and also they were much more wispy then the lower one in the pic. I'm not at all sure the 2 cases are the same phenomenon, in fact I'm skeptical that the lower cloud deck in Doug's pic is even a true anvil, but I had to mention that day because it did seem to be a real case of a double anvil.