I know this is not a proper answer to your question, but it reminded me of when I was little. Apparently, I used to ask my my mum & dad if I could have a 'rit' - because I assumed that one Ritz biscuit was called a rit!
Again, not an answer to the original question but you just remonded me, it's amazing how many people think appendix is plural and that you have one appendick on either side of your body.
Am I stating the bl''ding obvious (to paraphrase Basil Fawlty) to suggest that we are talking about a "Twick"!And btw I think you can get individual ones.
You can certainly buy individual Twix in 'fun size', usually with other Mars products like Bounty etc., but I think you can also get each product in a bag of each individual brand.