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
\usepackage{covington}
the command
\twoacc[\=|\u{a}]
or have the accents differently nested with
\twoacc[\u|\={a}]
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.
\documentclass{...}
and ending with\end{document}
.covington
package seems to do that.documentclass{article}\begin{document}
and\end{document}
around the code posted. The character does not show well here in SO either, since web browsers are not good at using multiple diacritics.