jQuery Simple Example

HTML

<script src="//www.addressfinder.co.nz/assets/v2/widget.js"></script>
<span>от</span>
<select name="num1" id="num1">
  <option value="1">1</option>
  <option value="2">2</option>
  <option value="3">3</option>
  <option value="4">4</option>
  <option value="5">5</option>
  <option value="6">6</option>
</select>
<span>до</span>
 <select name="num2" id="num2">
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option> 
<option value="5">5</option>
<option value="6">6</option>
<option value="7">7</option> 
</select>

JavaScript

$(document).ready( function() {
 $('#num1').change( function() {
  if( $('#num2').val() <= $('#num1').val() ) { 
  	$('#num2').val( parseInt($('#num1').val()) + 1 );
  }
 });
 
 $('#num2').change( function() {
  if( $('#num2').val() <= $('#num1').val() ) {
   	$('#num1').val( parseInt($('#num2').val()) - 1 );
  }
 });
});