Overlap to full-width sliders?

Hi ,

I am trying to make an overlap in the mobiel version,https://sintlaurensfonds.tydplayground.nl/
I would like the blue line on top from #slider-mobiel on top of the fitst image( with the boy =#full_slider_2), it is now behind the #full_slider_2.
I tried some things, can’t get it to work?
Cab you help me please?

Best regards,

This is not working:
/* overlap sliders op mobiel */
#full_slider_2 .avia-slideshow li:first-child, #full_slider_2 .avia-fullwidth-slider, #full_slider_2 .avia-slideshow li img {
position: relative;
visibility: visible;
z-index: 0;
}
#slider-mobiel .slideshow-3{

}
#slider-mobiel .avia-slideshow li:first-child {
position: relative;
margin-top: -50px;
visibility: visible;
z-index: 50000;
}