My trees endured -16F and even fruited. I know they should not have (Wilt Stop!). Peaches have wood damage at -18F so not sure what happened to your trees? -12F should not have killed them. I guess the early onset before they were hardened off did! I’m not sure a 4 in a hole would have done any better? My guess is not!
Good points made by all, thanks. I’d forgotten about the issue with patented varieties. That definitely is a plus in certain situations. I agree about the aesthetics as well. I prefer the look of multi-budded trees over n-in-1 plantings, particularly after leaf drop, although it’s not at the top of my priorities list.
Most of mine are in 4’ x 4’ square holes, four trees to a hole centered and spaced 18" apart. Some will be on their fourth and fifth leaf this year. Works great. Have to keep the center pruned out every year…not really a problem. Easier than dealing with a multi-trunk.
Not all are on the same rootstocks and I don’t really see an issue with that either (which varying mineral and water needs between rootstocks could theoretically be an issue, along with competition). I do have flood irrigation however.
They are grouped to a hole by species however (or by genus with Prunus sp.). I don’t see any for certain issues with different species, but it could make addressing nutrient deficiencies on one tree more difficult. Still in nature mixtures of trees get along so lets call it a guild. I think it will work but unless someone here already does it means you will be pioneering. (experimenting)
Can someone (Scott, Fruitnut and anyone?) show some pictures of their matured hedgerow fruit trees? I have a narrow un-fenced side yard that gets plenty of sun light whole day that may fit this strategy. I was a bit worried about the aesthitics aspect since it’s exposed to street, so is the curb appeal value…
Some pictures of the set up would be very nice and promoting ideas for invading the “curb appeal” without announcing the “fruiter” resident inside! 
Tom
Scott has a link in his profile to several pics of his orchard. I can no longer view the pics here at work due to security settings, but I think there are several that show what you’re looking for. I’m not sure @scottfsmith was considering curb appeal when he planted them, though. Orchard-Spring-2012 - Page 1