gistfile1.txt

<fieldset>

      <div class="form-group">
        <label for="exampleInputPassword1">Immobilientyp</label>

        <select name="category[]" id="type" class="form-control">
        {exp:channel:categories category_group="2" style="linear"}
          <option value="{category_id}"{if category_id IN ({low_search_category})} selected{/if}>
              {category_name}
            </option>
        {/exp:channel:categories}
        </select>
        </div>

        <div class="form-group">
        <label for="exampleInputPassword1">Kaufen oder Mieten?</label>
        <select name="category[]" id="variant" class="form-control">
        {exp:channel:categories category_group="1" style="linear"}
          <option value="{category_id}"{if category_id == low_search_category}{/if}>{category_name}</option>
        {/exp:channel:categories}
        <option value=""{if low_search_category == ''}{/if}>Category 1 or 2</option>
        </select>   
      </div>    

      <input type="hidden" name="require_all[]" value="category">


    </fieldset>