It is common for a camper to have different house battery charging sources:
Some solar installs will even have separate charge controllers running their own panel[s].
So what happens when multiple chargers are running at the same time? Nothing dramatic, as it turns out.1)
In general, the charging source with the highest voltage will “win”.
Example. Pretend you have a bank in a van with alternator charging and two solar panels on the roof. Each panel has a dedicated charge controller because the panels are of wildly different specs. This gives us three charging sources if you are driving down the road at dawn.
The alternator is just a plain alternator and puts out 14.0v all the time. The Morningstar charge controller is factory set to start Absorption at 14.4v.4) The Xantrex charge controller has been configured by you to start Absorption at 14.7v.5) 6)
Let's charge that battery. Here is the battery voltage along the way:
So how do we make them all charge all the time?
Barring heroic measures9) the alternator is going to stop contributing at some point during Bulk charging. That is ok, the alternator provided a ton of current while it was charging.
But if you want to the charge controllers (and some converters) can be convinced to stay roughly in sync and keep charging together. This is done by configuring the Absorption and Float setpoints on the controllers to be the same.10) There will be short periods where one controller makes a change before the other, but for most of the time both will be contributing at the same time.
Serial in this case means “one after another”. Sometimes the charging is done in a particular order for best results.
In most scenarios the high current source is either alternator charging or generator charging. A huge influx of power gets the battery off that deeply discharged point early in the morning. You will know when you have finished Bulk stage when the battery starts accepting less and less current. Since running the alternator or generator uses fuel and causes wear and tear, it's time to shut them down and use the next source.
The low current source is almost always solar. Solar is free to run and is not subject to wear and tear; it is perfect for the hours it will take to finish Absorption.