On the apple website, nothing is specified about what you need to install the 2 modules. They wrote that if you want to achieve the maximum of performance that install 2 of the same module:
but in this article there is a comment https://habr.com/ru/post/478346/#comment_20962254
where the guy says he installed 2 8Gb. I myself have been Proshka 2011 or 2012 and as far as I know there not officially supported 16GB of RAM.