I'd add code above the insert code to explode into the two variables like:
<?php
if (isset($_POST['dc'])) {
$valuesExploded = explode(",",$_POST['dc']);
$_POST["staff_id"] = $valuesExploded[0];
$_POST["dow"] = $valuesExploded[1];
}
?>
Then the code as you have it should just work.