The amount of current
that an alternator
charger creates is determined by the system voltage. You can run both DC sources simultaneously without harm. On my boat I have two alternators and two battery chargers and have had all four running at the same time on the same DC circuit without harm to either the batteries or any of the DC sources.
What will happen is as the system voltage rises, the DC sources will start cutting back on the current
(in amps) until the DC source with the highest voltage output (as determined by the regulator) will be sole source of current. The other DC sources will have cut their current output back to zero or near zero. Essentially, its something you don't need to worry about unless you have a DC source regulator
that allows too high of a voltage.
Its a whole different story with AC. In order to have more than one AC source, the phase must be matched.
That's one of the advantages of DC on a boat, you can have more than one DC source. Energy storage
using batteries is of course the other advantage.