You could separate them with a semi-colon, such as
<form METHOD="POST" action="uploadscript3.asp" name="fp1" onsubmit="return
checkCheckBoxes(this); myFunction();">
or why not just add a call to the second function inside of the first (at
the end of the function)? Ex:
function checkCheckBoxes() {
........ your code
myFunction();
}