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Try this:

@media (max-width: 767px) {
    #wpsp-123.wp-show-posts-columns {
        margin-left: -2em !important;
    }

    #wpsp-123.wp-show-posts-columns .wp-show-posts-single {
        width: 50%;
    }

    #wpsp-123.wp-show-posts-columns .wp-show-posts-inner {
        margin-left: 2em !important;
    }
}

Just replace 123 with the ID of your list.