Double ORDER BY in list of product in category view

[ExSpirit]

Which file should I modify to ignore administrator settings for ordering and use custom in PHP file? I would like to use double sorting - (first by GTIN (EAN,ISBN) and second if more products use the same value, by Availability Date).

[Milbo]

The correct solution is a search plugin, which can modify the query.

[ExSpirit]

But wouldn't search show different layout display and different URL,...? I would just like to modify the current structure by this small tweek, I just don't know which file to modify to do that (Joomla 3.4.3 and VM3).