One work that many hornists are interested to work on is the Weber Concertino, which contains multiphonics in the cadenza. How do you learn the technique?
One great answer is the new book by Michaeal Mikulka, A Practical Method for Horn Multiphonics. The book is available in PDF or hard copy versions. Text describes what you need to know, and there are a series of exercises and etudes with options for high and low voices. The section for composers is a great reference, and there is also a YouTube playlist with demo videos.
The book is available at the link below, where you can also access the YouTube playlist and a demo PDF:
The very end of the book is focused on the Weber Concertino cadenza. It is widely known, the cadenza, while it sort of works if you sing the top printed note and play the bottom printed note, does not really work exactly as Weber imagined. In this publication you will find five suggested versions of the cadenza for practical performance.
As those cadenza versions I think will be of particular interest to purchasers of this book, I have two specific comments to offer. First, the section begins with the passage as notated by Weber, but comparing with the original there are a couple misprints. I did not note other misprints in the book, but it would have been better not to have any in this critical passage.
The second specific comment is this. So, if you were working on the Weber and wanted to learn the multiphonics, how much would you spend on a lesson with a teacher that really understood how to play multiphonics? One that could help you work out a cadenza version that sounds good for you? The 117 page book is a little pricey at $18 for the PDF and $50 for hard copy (postage included), but with the market so limited and with so much specific, specialist information included that will be extremely helpful to you, it is worth the price.
For a bit more on multiphonics see this article in our Hornmasters series.