Best I can think of (and I have no idea if this is possible. It just sounded good as I thought of it now

) is to create a third calendar using google, 30Boxes or something like that - maybe even a hosted exchange service. Then use that to pull in the data from the two accounts you want and synch your device with the combined calendar.
Voila - Your two calendars are separate but your device contains them both.
You're going to ask how now, aren't you?