Have the first document load the second document when it's ready. With a onload handler on the first document that sets the location of the second iframe.
Let's say your iframes have 2 ids: "iframe1" and "iframe2". You load the database content in iframe1, and an empty page in iframe2.
Here is a sample code for the document loaded in iframe1 :
<html>
<head>
<script>
function init () {
frameElement.ownerDocument.getElementById("iframe2").contentWindow.location = "/someurl/document2.html";
}
</script>
</head>
<body onload="init()">
<!-- iframe1 content -->
</body>
</html>