I have personally noticed this only one time, however...
Baring any further mis-wired electrical connections, it is entirely possible that one alternator (although 'new') has a partially shorted field, and in so doing draws excessive field current, forcing the voltage produced by the faulty alternator to trip the overvoltage relay.
-OR--
The load paralleling wiring between the two voltage regulators is either incorrectly attached, or has a short to ground.
I would check the second option, first.
I personally own one of these 400-series twin Cessna aircraft, and they are a fine piece of flying machinery...if maintained properly (by someone who knows what they are doing).