While it won't give 2 additional images per row on mobile you could try this to give three images (as you've defined in your admin configuration) per row:

in responsive_mobile.css, in the blank line after line#388 add,

Code:
.additionalImages.centeredContent.back {
    float: left;
}
although I see there is similar code commented out at that line so you may have already tried and discarded this.