If they're both minor modes, then precedence is determined by the order of elements in minor-mode-map-alist
which, unless explicitly manipulated, is simply determined by the order in which the libraries were loaded.
Ensure that autocomplete
is loaded before yasnippet
, and yasnippet's minor mode map would have precedence.
You could also use eval-after-load
to adjust minor-mode-map-alist
after loading autocomplete
, to check for a yasnippet entry, and re-order the list if necessary.
(autocomplete
may use other methods, though. A temporary overriding keymap would still have precedence over the minor mode maps, for instance.)