I have handled more bales than I could ever count. Started when I was about 10 years old and always used one and have never seen anybody else use two.

Single hook - preferred the wood handle kind.

A family in our neighborhood used pitchforks -- they'd drop bales on the ground & then use pitchforks to toss them over the side of the box of  their grain truck  (yes that's lifting bales well over 6 ft. off the ground with a pitchfork).  The dad could pick up a full 50 gallon oil drum and load it onto the bed of his truck.

I found that real tight wrapped square bales, them green heavy ones that start at 80 pounds and go up, one could use two hooks.  Normally with our little round bales, only one was needed.   I picked up all our hooks when things shifted to the big bales,  got them in the barn.  What was someone saying about stacking hay to the roof of the barn, on a hot day?   Think it got close to 130 degrees up there in the afternoon, and I wonder what is wrong with me now. 

We always used one.  The guy pulling the bales from the bailer to the wagon used it.  Never used one stacking bales on the wagon or in a barn.

  Jim