for example you can put something like that between <head> </head>
part of the code
....
<script src="http://maps.google.com/maps?file=api&v=2&key=<?
$adres_url=$HTTP_SERVER_VARS["SERVER_NAME"];
if(ereg('balticart.lv',$adres_url)) {echo"FIRST API KEY";}
elseif(ereg('balticart.com',$adres_url)) {echo"SECOND API KEY";}
else {}
?>"
type="text/javascript"></script>
<script type="text/javascript">
//<![CDATA[
function load() {
if (GBrowserIsCompatible()) {
var map = new GMap2(document.getElementById("map"));
....