I stick the tapered nozzle I use when blowing up snow tubes, pool floats, etc and I stick it between the sides and give it a blast of air. Just to add ; I do this dozens of times each spring when I start tapping maple trees and need to get my buckets apart.
Put a piece of baling twine in the bucket before stacking I usually have to pull several apart each year. Also putting ratchet strap around bottom bucket holding it to post or something similar allows you to pull harder- also a pair of vise grips or pliers on top handle help you to pull apart. Or you can run hot water over bottom bucket to expand bucket and air pocket, allowing top bucket to slip out. Charles
Hold the buckets upside down and take a hammer and tap the rides on the lowest bucket. Pops right off
Success! I told my husband about all the various suggested methods, and here's his: put them near the wood stove for a little while to warm them up. It worked for the three, but the yellow & white ones were still stuck. So I banged on those with a hammer, and that did the trick. Now I can clean 'em up and use them for branch nubs and kindling scraps.
Exactly! Squirt a hose on the lip of the bottom bucket, water will work it's way into the bottom bucket and pop the upper bucket out/off.