Fijate si te sirve algo así:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>t</title>
<script>
//aquí determinás el orden de carga
var ifrs=[{url:'http://www.google.com',w:300,h:300,pNodeId:'uno'},{url:'http://www.forosdelweb.com',w:500,h:200,pNodeId:'dos'}]
function cargarIframe(){
if(ifrs.length<1)return;
var i=document.createElement('iframe');
i.width=ifrs[0].w;
i.height=ifrs[0].h;
i.src=ifrs[0].url;
if(!window.ActiveXObject)
i.onload=cargarIframe;
else
i.onreadystatechange=function(){
if(i.readyState=='complete')
cargarIframe();
}
document.getElementById(ifrs[0].pNodeId).appendChild(i);
ifrs.splice(0,1);
}
onload=cargarIframe;
</script>
</head>
<body>
<p id="uno"></p>
<p id="dos"></p>
</body>
</html>