If you are using Flexbox (bootstrap), assign an ID to the div that will disappear and assign the col-12 class (for mobile resolutions) and col-md-6 (for resolutions greater than average) to the div should occupy the 12 columns, and col-6 for the div that should disappear.
<div class="container">
<div id="identificadorDaDiv" class="col-6">
Essa desaparece quando chegar em md
</div>
<div class="col-12 col-md-6 ">
Essa ocupa todas as 12 colunas quando chegar em md
</div>
</div>
In your CSS, you can add an @media query for medium or smaller resolutions (Bootstrap default is 768px)
@media screen and (max-resolution: 767px) {
#identificadorDaDiv {
display: hidden
}
}