Trouble changing to secondary featured-image on hover

hi there! i’ve been trying to change the thumbnails of my post’s featured images to the secondary featured image on mouse-hover.

installing the multiple post thumbnail plugin worked fine, i can select a secondary image but i’m having trouble implementing that hover option/effect.

SUMMARY:
-theme: Salient
-plugin for secondary featured image: Multiple Post Thmbnails
-tutorial i tried to use: https://www.scratchinginfo.net/hover-two-featured-images-wordpress-via-multiple-post-thumbnails-plugin/
-link to my texting-site: https://feeluna.com/offline/
-problem: when i hover over one of the thumbnails, i want the secondary image to appear, currently only the posts named ‘TEST…’ have a secondary thumbnail selected.

i tried inserting the bits & script as the tutorial above says, but it seems i need a custom solution for my theme…since i’m not allowed to post my themes code, it would be helpful if someone could check out the link to our test page i posted above and see via developer-tools how this could be done.
…we would be grateful for help and would like to pay you back for the favour!