It sounds to me like your regulators need to be adjusted. S/N 1399 and on are different from the older models in that instead of one regulator running 2 alt. each alt. uses one regulator. Check the voltage output from each alt. separately. It should be 27.5 +/- .2 volts. with landing light on. If they are out of adjustment, one will kick the other offline.

This was a common problem with some of the C402's I used to work on.