I was cooking some hot dogs for the kids over the weekend, and I pulled the first layer of four buns out of the wrapper, pulled the buns apart, and put them on the plates. My seven-year-old son, noticing that the buns were baked together and had to be pulled apart, quite sensibly asked, “How come the ones in the middle are already cut open?”
Got me.
Dr. Watson
“There is no such thing as a free lunch.”
They are sliced from the side.
Slicing them from the side would work for the outer two buns, but there are four baked together. How do they slice the two in the middle?
Dr. Watson
Aint sliced. They baked with a piece of metal in them so they have two slices to give an impression of slicing.
A piece of metal? If that’s the case, why doesn’t the baked dough appear the same on the “sliced” interior surfaces as it does on the bottom of the bun? I’d buy that they’re sliced from the side (slicing direction paralell to the long axis of the hot dog bun, Crick), but I have no proof.
There are sliced from the ends, inward. The other side. 
Like with four (or five for the 10 packs) knives at once poking into the bun from the ends.
What SoMo said.
I should qualify myself by stating I worked a couple of years in a production bakery. Though I worked mainly on the loaf bread side, I occasionally would relieve the bun guys for lunch.
The process for hot dog rolls is.
Dough is rolled into a long “snake”, this is cut into little hot dog sized lengths and dropped into pans. Each pan contains 20 buns.
The pans then go into the proofing area where they are allowed to rise. This is when the dough mushes together to form the huge brick 'o buns you are familar with.
After proofing the pans are fed into the oven. When the baking cycle is complete the bread is “de-panned” and placed on racks to cool.
Once they are cool enough to cut, they are fed by conveyer into a machine called, of all things, a bun slicer. It resembles a chain saw with long fingers. (sorry best description I can come up with).
Picture the bun bricks passing through lengthwise while the fingers whirl around and poke the buns from the end, individually slicing each bun and leaving enough bread on the “bottom” to keep the hot dog from falling out.
The buns then travel by conveyer to the bagging area where, you guessed it, they are broken into the proper size and bagged.
I mention the bagging part so I can let you in on a little secret. The twist ties are color coded so the route people can pull stale packages. A bag with a different color tie (or clip) on it was probably missed by the delivery guy the last time he pulled old stock.
Amazing. Every day learns I a new something.
Thanks.
Dr. Watson