one possible fix:
i had a similar issue. two iCloud accounts (one for my appleID and one for my migrated MobileMe account) and one set of devices - iPad, iPhone, mac.
i got everything set up the way i wanted first, sans content - basically i wanted all devices synced through the iCloud account so that i could ditch the secondary account.
i went ahead an added the second account to the mac, did a select all of the calendar events, then dragged onto the desktop. then i created an "Old" calendar on my primary iCloud account calendar.
finally, i doubled clicked the .ICS file on the desktop and told it to add all those events to the "Old" calendar. then i deleted the secondary account.
only downside was losing all the calendar designations for those old events. so that requires some manual entry/changes.
i'd love to hear if someone found a way around that bit.