Greetings,
I have sometimes heard it asked if it's possible to play a complete and true trill in octaves. I believe I have found a way of doing this, by using the fingering of 2&5, alternating with 1&4 (in the same manner in which one would typically perform trill in sixths).
Here is a link to a demonstration of this technique:
Attached also is a graphic of a preliminary exercise, showing how it works.
I've heard that Henselt used to play Chopin's Black Key Etude in octaves, and suspect he may have used a similar approach....
Anyone tried this approach or technique before?