a fast way to join NLA strips? In the NLA editor, splitting strips is just a matter of one key press.
I did not find a similarly easy way to join them again (except ctrl-z :D)

So far, what I usually did, was:


copy the last strip for reference
delete all but the first strip
adapt the length of the strip


If the strips belonged to two actions, it would even involve joining the actions by copy-paste and careful "trimming" of each action before.

Obviously, this cannot be the way how it's meant to be done :)

So my question is: How to do it correctly / efficiently and beginner-friendly?

(apologies if there is an answer to this question here. I searched the web for hours yesterday, but didn't find any)
There is a menu option and keystroke Ctrl-g to create a Meta strip that groups multiple strips.

It sounds like you want to undo the splitting of a strip, but that same operation also has to have a defined behavior if the two strips selected were not originally split from the same source.  

The logic to detect that two strips were originally split from the same source would be non-trivial and have a fairly long set of test cases for a QA team to perform.