I have two PHP scripts to be called simultaneously:
Your problem is as simple as you can imagine. You just don't realize it maybe for a bit lack of knowledge of HTML. So Your code is ok and everything is working as you want but the script that should run at the same time isn't, what is the problem?
This up here is your problem. The onload call only takes place when everything inside the body tag is completely loaded. So, as your iframe is inside the body the html interpreter load everything (including the iframe and its source), then call your callScripts function.
To solve your problem I recommend you to create your iframe inside your script. Would be something as this:
Title of the document
Let me know if it work after that