In my headcannon, you are able to put a decapitated fluffy pony head, onto a living fluffy pony, just cut a fluffy ponies head and quickly stitch it onto another fluffy pony that’s alive, however you need to stitch it on extremely QUICKLY before the brain of the decapitated fluffy shuts down completely, making the head a useless cosmetic for the alive fluffy, it may seem cool but there are downsides to doing this, the 2nd head has a very high risk of becoming derped, or being brain-dead, or just having brain damage, so its extremely rare to have a fluffy pony with a fully functional 2nd head.
The reason why this happen is unknown, but people think that it might be the genetics fucking up a bit.
Also the process can be faster if you make a huge wound on a fluffy pony and put the decapitated head over that wound, then either stitch it or wrap a bandage around the neck of the 2nd head.