A reason not to run both chargers at the dock on shore power is that together they can draw a good deal more than the 16 amp rating of the shore power supply.
This only happens if the batteries are drained enough to be able to accept that much current. But even the 100 amp charger by itself can draw over 13 amps if it is really pushing out a full rated 100 amps at 28 volts. That doesn't give you a lot of room to run anything else. If the water heater comes on, you'll trip the breaker, for example. Hence, the reason for the smaller charger for use on shore power.
Most of our generators have the "oomph" to run both chargers, and more.
But remember, as the batteries start to get over about 80% charged, the rate at which they can accept amps starts to decline pretty quickly. If your 100 amp charger is not putting out 100 amps, adding a second charger really won't help.
A lot of these problems have gone by the wayside with newer, smarter inverter/chargers which are clever enough to reduce the charging current if other things need the power.
North Bimini, Bahamas