Finally found a good dual audio copy here:

https://webeandola.wordpress.com/2016/09/09/descargar-the-secret-life-of-pets-hd-720p-ingles-subtitulada-mega-1fichier/

(The Mega link works)

I almost discarded it as fake, but the password printed on that page did work to unlock the rar.

If you thought searching for English language torrents was a minefield of scams and fake links, searching for Spanish language torrents is 100x worse.

I'd settle for something with a good audio stream even if the video is poor  - I can mux with video from an English copy.

**Update:** found

[Request] "Mascotas 2016" - which is the Spanish language version of "The Secret Life of Pets".