const $slider1 = $('.slider-nav').slick({
...
}).on('beforeChange', function(e, slick, currSlide, nextSlide) {
$slider2.slick('slickGoTo', nextSlide + 1);
});
const $slider2 = $('.slider-for').slick({
...
}).on('beforeChange', function(e, slick, currSlide, nextSlide) {
$slider1.slick('slickGoTo', nextSlide - 1);
});