I was just reading about Bilbo and the three trolls: William, Bert, and Tom.

At the end of the quote, there is a reference to multi-headed trolls.

William choked. "Shut yer mouth!" he said as soon as he could. "Yer can't expect folks to stop here for ever just to be et by you and Bert. You've et a village and a half between yer, since we come down from the mountains. How much more d'yer want? And time's been up our way, when yer'd have said 'thank yer Bill' for a nice bit of o'fat valley mutton like what this is." He took a big bite off a sheep's leg he was toasting, and wiped his lips on his sleeve.

Yes, I'm afraid trolls do behave like that, even those with only one head each.
- The Hobbit p.35

Does Tolkien further describe multi-headed trolls in his notes, or is the reference just a one off limited to The Hobbit?