Add multiple window.onload events

In my ASP.NET User Control I'm adding some JavaScript to the window.onload event:

if (!Page.ClientScript.IsStartupScriptRegistered(this.GetType(), onloadScriptName))
  Page.ClientScript.RegisterStartupScript(this.GetType(), onloadScriptName, 
    "window.onload = function() {myFunction();};", true);            

My problem is, if there is already something in the onload event, than this overwrites it. How would I go about allowing two user controls to each execute JavaScript in the onload event?

Edit: Thanks for the info on third party libraries. I'll keep them in mind.