Which Lego Technic sets are officially combinable?
Whenever a LEGO Technic set is officially meant to be combinable with another set, it is mentioned right on the box, as well as in the instruction booklet, and also in the official set description at the Online LEGO Shop. The combined instructions are posted at the LEGO Technic website.

As far as a list, I don't know of any place that has a comprehensive list like that, but I agree it would be very useful. I do know, however, that the two LEGO Technic Pull-Back Racers released each year since 2013 can always be combined with each other with downloadable instructions. Here is the list of the sets:  https://brickset.com/sets/list-15258