See the Pen github.comselect2/select2-bootstrap-theme/issues/35 by Florian Kissling @fk on CodePen.
<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.