You don't have to do anything. They wont "fight" each other. All a charger knows is the systems voltage. It will regulate the current
or whats sometimes called "amperage", until the voltage comes up to the level that the charger or alternator was designed to bring the voltage up to. Basically, DC sources regulate voltage by either increasing or decreasing the current
(in Amps) they produce.
If you have more than one source of DC, which you do. As the system voltage rises, the DC sources with the lower maximum voltage settings, will start cutting back their current flow. When the system voltage exceeds their maximum voltage output, they will cut their current output to zero or near zero. The DC source with the highest maximum voltage setting will be the one remaining device that is keeping your system voltage at its present voltage level.
None of your DC sources will ever have exactly the same maximum voltage or produce the same current at the same RPM(as applied to alternators)....even if the specs say they do. I have two identical alternators which charge the same circuit and even when run at the same speed, they never produce the same exact current. I have two digital ammeters, one for each alternator, where I can read each alternators current output. My two chargers never produce the same current at the same system voltage.
There are smart three stage chargers, but you will do no harm to them by running the engines alternator while they are on. Alternately, the three stage charger(s) will not damage your alternator(s) while they are turning. You're three stage chargers just wont be able to do their three stage function while the alternators are turning, which won't hurt either the alternator or the charger.
I start my engines all the time while connected to shore power
with both battery chargers on. There have never been any problems in doing this. I have two chargers and two alternators...all four sources of DC been operating at the same time with no ill effects.
Unlike some siblings, my four DC sources get along just fine 100% of the time.
Its AC where you will have major problems if you every try combining two different AC sources...like your shore power with your AC genset or with you're inverter
...or try combining any combination of the three sources. Combining AC sources can be done with the right equipment
by matching the wave form, but how that is done would be going outside the scope
of your question since yours is a question about DC.