Assumes at least 4 Testimonials are <= 750 characters. Doesn't get 2 random, sort is always the same.

TestimonialCall.php

    public function getTestimonials(){
        return Testimonial::get()
            ->filterByCallback(
                function($item, $list) {
                    return strlen($item->Description) <= 750;
                }
            )
            ->sort('RAND()')
            ->limit(2);
    }

Solved: ->sort('RAND()') needs to come before ->filterByCallback()