Most light twins have voltage regulators which are connected together electrically, as one alternator starts to take more load than the other the regulator on the heavier loaded side ups the current to the lighter loaded side regulator & make it take a bit more of the load, hence balancing the load between the two.
That sort of system is definately used on Pipers like the Seneca & the Aztec. Balancing can be a nightmare if one or other Alternator has been changed, this tends to "upset" the system because the internal resistances of the old & the new alternators may be different. I had this once & the only way to get them balanced was to get another alternator from the same manufacturer for the other side.
Beechcraft tend to use a different system, althought they have 2 regulators one is a spare used only if he other goes U/S. The only way you can get the alternators to balance is to have completely equal resistance in the left & right system any high resistance connections will cause the other side to take more of the load.