a common method from that era was to connect each alternator output to an amp gauge at the helm station, then back to the starter lug. #10 wire was often used which resulted in a significant voltage drop -- the alternator might be putting out 14.6 v but the battery is only seeing 13.8 v, which makes for low charging amps and long charging times. Effectively, the alternator thinks the battery is fully charged but the battery is really only about 70% charge.
In twin-engine installations 2 battery switches of the "off-1-both-2" variety were used so either battery could start either engine, or either engine could charge either or both batteries.
Some of the taiwan trawlers with lehmans had a variation on this. Both starters were connected to a common terminal, and 2 "off-on" battery switches were used to connect either or both batteries to the starters. The house distribution panel was connected to an "off-1-both-2" switch so either or both battery banks could be used to provide power for the lights, refrigerator, etc.
Now a common method has one engine equipped with a high-output alternator connected directly to a large battery bank for the house loads, and the other engine has the original alternator charging a starting battery that provides power for starting either engine. An "off-on" battery switch can be used to connect the two battery banks if needed, to either use the house bank to start or use the other alternator to charge the house bank.
Connecting the alternators directly to the battery, battery buss, or starter lug with a #4 or #6 cable will significantly improve charging capability. This bypasses the amp gauge at the helm station, but that gauge can be replaced with a voltmeter.
A fuse at the battery buss connection is a good idea to protect against a short circuit.
