Select que cambian al mismo tiempo

[America|UNK]

Asignando el valor al nodo hermano.

Código HTML:

<select name="select" id="select" onchange="this.nextSibling.nextSibling.value = this.value">
  <option>Seleccionar</option>
  <option value="1">aaaaa</option>
  <option value="5">bbbb</option>
  <option value="7">ccc</option>
  <option value="3">ddd</option>
</select>
<select name="select2" id="select2" onchange="this.previousSibling.previousSibling.value = this.value">
  <option>Seleccionar</option>
  <option value="1">aaaaa</option>
  <option value="5">bbbb</option>
  <option value="7">ccc</option>
  <option value="3">ddd</option>
</select>