PHP - llenar dos select con una misma consulta en mysqli

$select_1 = '<select name="select_1" id="select_1" >';

$select_2 = '<select name="select_2" id="select_2" >';

while( $option = mysql_fetch_array($result)) {

 $opcion = "<option  value=".htmlspecialchars($option[cid']).">".htmlspecialchars($option['nombre'])."</option>";

 $select1 = $select1 . $opcion;

 $select2 = $select2 . $opcion;

}

echo $select1;

echo $select2;

<?php

include_once 'database.php';

$select1 = "" ;

$select2 = "";

while($row_unidades = mysqli_fetch_array($unidades)){

   $option = "<option>" . $row_unidades['idunidad'] . "</option>";

   $slect1 = $select1 . $opcion;

   $select2 = $select2 . $opcion;

}

?>

<select><?php echo $select1; ?></select>

<select><?php echo $select2; ?></select>