on your page, you will see a script line that looks like:
<script type="text/javascript" src="http://maps.google.com/maps?file=api&v=2&key=ABQIAAAAvZMU4-DFRYtw1UlTj_zc6hT2yXp_ZAY8_ufC3CFXhHIE1NvwkxQcT1h-VA8wQL5JBdsM5JWeJpukvw">/*wagmp*/</script>
this is the line that passes the api key.
you would need to create a php statement to determine which api key should be used based on the $_SERVER["HTTP_HOST"]
<?php
switch ($_SERVER["HTTP_HOST"]) {
case "www.domain1.com":
$key = "key for domain 1";
break;
case "www.domain2.com":
$key = "key for domain 2";
break;
default:
$key = "key for domain 1";
break;
}
?>
<script type="text/javascript" src="http://maps.google.com/maps?file=api&v=2&key=<?php echo $key; ?>">/*wagmp*/</script>