Using @Bernard’s hint, I checked the
covington package, and its documentation says: “LATEX doesn’t give you a convenient way to put two accents on
the same letter” and introduces some macros for the purpose. There are special macros for combining acute, grave, or circumflex with macron, and a more general macro
twoacc for combining two accents, with a peculiar syntax: “its arguments are in square brackets, not curly brackets, and are separated by |. The ﬁrst argument is the upper accent (only) and the second argument is the letter with the lower accent indicated.” So in your case you could use, after
or have the accents differently nested with
This works, but especially the first one does not give a very impressive result (the diacritics are too far from each other):
(In Latin grammars and dictionaries, the latter form seems to be more common, when indicating that a vowel may be either long or short. And the result for it looks actually better than in my old Latin books.)
The package is from year 2001, so there might be better tools for the purpose now.