Bootstrap and select2 how to combine two selects in input-group?

Question

Bootstrap

Hello.
Just two selects in bootstrap are connected. And they don't want to connect select2, how to fix it?

<div class="input-group">
   <select class="custom-select">
       <option></option>
    </select>
   <select class="custom-select">
       <option></option>
    </select>
</div>