Just thinking out loud.....
Frame A:
Does what it has to do. When it's finished it adds a unique ID to a file/db
(maybe session ID)
Frame B:
Checks to see if it's session ID is in the file/database. If it's there, that means that Frame A is loaded and Frame B continues.. else it redirects to itself and checks again..and again..again...until the session ID is found in the DB
Issues (maybe) :
Infinate loops
Creating the unique ID
Will have to clean up the DB every so often
The people who pay me insist that I actually work for my money, so I don't have time to slap this one together.