Interdependent date fields – is it possible?

I would like to have two date fields for events: one for the actual date when the event occurs, and another to indicate when the room is occupied (for preparations, cleaning up, etc.). Ideally, the second field should be identical to the first unless something else is indicated.
Is it possible
(a) to populate both fields with the same data when the event is created?
(b) to modify the second field at creation time?

So, the scenario is:
1. I create an event, set a date/time, save, and both the date fields are identical.
2. I create an event, set the actual time, set an occupied time (and it is checked that this span is larger than the first).

I hope this is clear.