1. Select source calendar in the Calendar List > File > Export > Export > Save ICS file > Delete original calendar if desired
2. Double click ICS file > Select a destination calendar > OK

    tell application "iCal"
        repeat with anEvent in (get events of calendar "A")
            move anEvent to (end of events of calendar "B")
        end repeat
    end tell

and the second from hubionmac:

    tell application "iCal"
        set allEvents to events of calendar "A"
        repeat with theEvent in allEvents
            copy theEvent to end of events of calendar "B"
            delete theEvent
        end repeat
    end tell

/mac | Oct 14, 2012