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I don’t know if it’s the correct or recommended way, but here’s what I’ve worked out after a lot of trial and error. Following this should allow you to charge without tripping any breakers. It’s important to note, you can only go up to half speed (340 RPM on the propeller shaft) while also charging. This probably means charging while running one or both engines on half speed is the most common use case, so I’ll describe the procedure for that scenario.

1. Make sure electric engine motor power switch is set to off.
2. Set diesel engine a little under great speed, which is 435 RPM on the prop shaft. So, I usually do about 400, to give a little buffer.
3. Set motor direction to forward.
4. Set motor switch to parallel.
5. Set battery switch to series.
6. Set speed control dial full right / low power.
7. Turn motor power to on.
8. Go look at the motor current gauge on the speed control area. You should see the needle move to the left toward the red line, but not cross it, and not trip the breaker. If the breaker trips, double check everything, and possibly reduce diesel a bit to give more buffer.
9. Adjust the speed control wheel toward higher power, until you reach 340 on the shaft rotation guage. Always keep an eye on motor current to ensure you don’t redline it.
10. If the propeller shaft is turning slower than 340, go tune the cylinders and carefully bring up power on the diesel. Note that there’s also a motor current guage for each engine in the overhead guage panel by the diesels. Keep an eye on that whenever making any adjustments, including tubing cylinders, to make sure you don’t exceed redline.

Procedure is essentially the same for charging at speeds lower than half, except the settings on the series/parallel switch need to be adjusted.
Half speed: diesel full speed, motor parallel, battery series
Slow speed: diesel half speed, motor parallel, battery series
Dead slow: diesel slow, motor parallel, battery parallel
Diesel not propelling boat: diesel dead slow, motor series, battery parallel.