damn! I take it you want to save them both. if you dont just drill a hole through the bottom of the bottom bucket to release the suction and they should come apart a bit easier.
I find the best method when wanting to save both buckets is to get someone to hold the bottom bucket on the ground while you twist the top out. the other thing to try is to poor freezing cold water into the top bucket which will casue it to shrink a tiny bit and this might just help

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