[Sorting by multiple meta keys] is something that WordPress added support for in 4.2: https://make.wordpress.org/core/2015/03/30/query-improvements-in-wp-4-2-orderby-and-meta_query/

Please note: It is not so obvious, but for ordering with multiple meta keys you have to give each meta_query a name and then use these names in the orderby.

Source: How to order by multiple meta keys?.