YOu can already see this but here's what they posted as their reply:

It's handled automatically. In the Setup dialog the user can set their "Target Subexposure". The camera will then automatically adjust adjust the number and duration of subexposures to get as close to that as possible, while achieving the desired total exposure time.

For a detailed explanation please see Page 21 of the user manual:
WooHoo! This is a first and very very cool.