<select class="form-select form-select-solid mb-2" name="regroupements" data-control="select2" data-placeholder="Select something" data-allow-clear="true" multiple>
 <option></option>
 {% for niv1 in niveau1 %}
 <option value="{{ niv1.id }}">{{ niv1.caption }}</option>
 {% for niv2 in niveau2 %}
 <option class="l2" value="{{ niv2.id }}">{{ niv2.caption }}</option>
 {% endfor %}
 {% endfor %}
 </select>

.l2 {
 padding-left:20px !important;
 }