That kind of explanation would be too much to do here on this forum as you would probably need Presstalk to handle the overflow.
Basically, you cover page would be on its own form page, the check as well as would the overflow.
Both the check and the overflow form pages would use an overlay that would encompass all that is static.
Now each form page would be setup with its own tray call. That is something you setup in the
Page setup->Paper handling or the
Page setup->Advanced paper handling section.
You need to have the proper PPD, as to get the proper Postscript command.
I suggest you open a technical support ticket through our
website. A technician will be able to guide you toward your goal.