<div class="input-group select2-bootstrap-operator">
      <select class="form-control select2-single" tabindex="-1" aria-hidden="true">
        <option></option>
        <optgroup label="Alaskan/Hawaiian Time Zone">
        <option value="AK">Alaska</option>
        <option value="HI" disabled="disabled">Hawaii</option>
      </optgroup>
      <optgroup label="Pacific Time Zone">
        <option value="CA">California</option>
        <option value="NV">Nevada</option>
        <option value="OR">Oregon</option>
        <option value="WA">Washington</option>
      </optgroup>
      <optgroup label="Mountain Time Zone">
        <option value="AZ">Arizona</option>
        <option value="CO">Colorado</option>
        <option value="ID">Idaho</option>
        <option value="MT">Montana</option><option value="NE">Nebraska</option>
        <option value="NM">New Mexico</option>
        <option value="ND">North Dakota</option>
        <option value="UT">Utah</option>
        <option value="WY">Wyoming</option>
      </optgroup>
      <optgroup label="Central Time Zone">
        <option value="AL">Alabama</option>
        <option value="AR">Arkansas</option>
        <option value="IL">Illinois</option>
        <option value="IA">Iowa</option>
        <option value="KS">Kansas</option>
        <option value="KY">Kentucky</option>
        <option value="LA">Louisiana</option>
        <option value="MN">Minnesota</option>
        <option value="MS">Mississippi</option>
        <option value="MO">Missouri</option>
        <option value="OK">Oklahoma</option>
        <option value="SD">South Dakota</option>
        <option value="TX">Texas</option>
        <option value="TN">Tennessee</option>
        <option value="WI">Wisconsin</option>
      </optgroup>
      <optgroup label="Eastern Time Zone">
        <option value="CT">Connecticut</option>
        <option value="DE">Delaware</option>
        <option value="FL">Florida</option>
        <option value="GA">Georgia</option>
        <option value="IN">Indiana</option>
        <option value="ME">Maine</option>
        <option value="MD">Maryland</option>
        <option value="MA">Massachusetts</option>
        <option value="MI">Michigan</option>
        <option value="NH">New Hampshire</option><option value="NJ">New Jersey</option>
        <option value="NY">New York</option>
        <option value="NC">North Carolina</option>
        <option value="OH">Ohio</option>
        <option value="PA">Pennsylvania</option><option value="RI">Rhode Island</option><option value="SC">South Carolina</option>
        <option value="VT">Vermont</option><option value="VA">Virginia</option>
        <option value="WV">West Virginia</option>
      </optgroup>
      <option value="TNOGZ" disabled="disabled">The No Optgroup Zone</option>
      <option value="TPZ">The Panic Zone</option>
      <option value="TTZ">The Twilight Zone</option>
		</select>
      <div class="input-group-addon">=</div>
      <select class="form-control select2-single" tabindex="-1" aria-hidden="true">
        <option></option>
        <optgroup label="Alaskan/Hawaiian Time Zone">
        <option value="AK">Alaska</option>
        <option value="HI" disabled="disabled">Hawaii</option>
      </optgroup>
      <optgroup label="Pacific Time Zone">
        <option value="CA">California</option>
        <option value="NV">Nevada</option>
        <option value="OR">Oregon</option>
        <option value="WA">Washington</option>
      </optgroup>
      <optgroup label="Mountain Time Zone">
        <option value="AZ">Arizona</option>
        <option value="CO">Colorado</option>
        <option value="ID">Idaho</option>
        <option value="MT">Montana</option><option value="NE">Nebraska</option>
        <option value="NM">New Mexico</option>
        <option value="ND">North Dakota</option>
        <option value="UT">Utah</option>
        <option value="WY">Wyoming</option>
      </optgroup>
      <optgroup label="Central Time Zone">
        <option value="AL">Alabama</option>
        <option value="AR">Arkansas</option>
        <option value="IL">Illinois</option>
        <option value="IA">Iowa</option>
        <option value="KS">Kansas</option>
        <option value="KY">Kentucky</option>
        <option value="LA">Louisiana</option>
        <option value="MN">Minnesota</option>
        <option value="MS">Mississippi</option>
        <option value="MO">Missouri</option>
        <option value="OK">Oklahoma</option>
        <option value="SD">South Dakota</option>
        <option value="TX">Texas</option>
        <option value="TN">Tennessee</option>
        <option value="WI">Wisconsin</option>
      </optgroup>
      <optgroup label="Eastern Time Zone">
        <option value="CT">Connecticut</option>
        <option value="DE">Delaware</option>
        <option value="FL">Florida</option>
        <option value="GA">Georgia</option>
        <option value="IN">Indiana</option>
        <option value="ME">Maine</option>
        <option value="MD">Maryland</option>
        <option value="MA">Massachusetts</option>
        <option value="MI">Michigan</option>
        <option value="NH">New Hampshire</option><option value="NJ">New Jersey</option>
        <option value="NY">New York</option>
        <option value="NC">North Carolina</option>
        <option value="OH">Ohio</option>
        <option value="PA">Pennsylvania</option><option value="RI">Rhode Island</option><option value="SC">South Carolina</option>
        <option value="VT">Vermont</option><option value="VA">Virginia</option>
        <option value="WV">West Virginia</option>
      </optgroup>
      <option value="TNOGZ" disabled="disabled">The No Optgroup Zone</option>
      <option value="TPZ">The Panic Zone</option>
      <option value="TTZ">The Twilight Zone</option>
		</select>
    </div>
Preview

See the Pen github.com/select2/select2-bootstrap-theme/issues/35 by Florian Kissling (@fk) on CodePen.