drawing current from multiple sources in parallel
Theoretically you will only get a current reading from the bike with the highest output Voltage (assuming each bike's current is sensed seperately), all other bikes will contribute zero (or close) current.

In reality the current will most likely split in some way because the output voltage of the bike contributing the hihest current will collaps the most.

For battery charging you still should use some kind of charge electronics.
(eg. all bikes feed an intermediate circuit which feeds a buckboost battery charger)