Many vehicle designs have a single bus that carries the current to the three systems. Using mosfets, or in the simple case - diodes, you can switch current onto the bus, or shunt it off into the system you're dealing with. Typically in a dumb system the bus is kept at voltage within a certain range, but that voltage is allowed to move up and down within that range, and each system acts differently based on the bus voltage.
Taking each system one at a time:
Alternators
The alternators attempt to keep the voltage at, say, 52V. As long as it's at or near 52V, they regulate their current output so the bus stays between, say, 50 and 52. When the voltage is below 50V, they run full tilt at 250A to get it back up there. That's their only job, since they never take current from the bus.
Motors
The motors don't care about the bus voltage. The throttle shunts current from the bus to the motors, and the bus voltage drops if the other systems aren't adding current quickly enough.
Batteries
The batteries are the more complicated system. If the bus voltage is 50V or higher, they take current from the bus, and charge the batteries. Just activate the 0.5A charger when the bus voltage is above 50V. 50V is the signal that the alternators have excess capacity, and the batteries can charge. They do not put any current onto the bus.
If the voltage is 48V to 50V, the batteries disconnect themselves from the bus if they aren't already supplying current. If they are supplying current, they continue to do so, trying to keep the bus voltage above 48V, but below 50V.
If the voltage is under 48V, the batteries start trying to force current onto the bus, keeping it between 48V and 50V.
The batteries only do this, of course, until they run to about 50% of their capacity, then they stop supplying current to extend the life of the batteries.
Result
When the motors draw no current, the alternators are running on a light load, charging the batteries.
When the motors are drawing 250A or less, the alternators are running at full load, and the batteries are disconnected.
When the motors are drawing over 250A, the alternators are running at full load, and the batteries are fully connected to the bus, pushing as much current as they have left onto the bus.
Reality
This system has its faults. If the battery-->bus or alternator-->bus regulators aren't happy with each other, you'll end up with system oscillation which might not be easily resolved.
Most modern systems use active elements and a single bus controller that tells each system what to do. This controller knows how much current the motor is commanding, and tells each subsystem how much current to supply, while monitoring other bus and system vitals to prevent overvoltage, overcurrent, and overtemperature violations.
But with careful design and tuning, the dumb system can work, and works well. An advantage is that the system can handle more devices either providing or using current without having to specifically communicate with a central controller. If you add solar panels, or a cabin cooling system, they hook directly to the bus with a little logic that turns them on or off depending on the bus voltage. On a central bus system you will likely have to reprogram the central controller and may even have to have these devices communicate with it so it doesn't accidentally discharge the batteries when it believes it is in a charge state, for instance.
Best Answer
I'd strongly consider this (below) as a design: -
The supply above produces a 5v output but by tweaking the ration of R1 to R2 you can produce 6V.
There are plenty of options for building your own. See this link.