Have a look at the code posted in this thread:
http://www.codingforums.com/showthread.php?t=285082
basically, the idea is that when the first date is selected the minDate for the second date is set via options.
No need for validation or blocking the form submission - the user just can't pick a date that doesn't make sense.
For extra security, you can make those inputs read-only. The date picker will still work on them, but you won't be able to type values in from the keyboard
oh, yes - and as Andrew points out, the datepicker will only work using the IDs of the inputs - class names are very problematic