The alternator won't "load" without initial input voltage to the exciter windings.
It won't drain the battery if it's not running due to the polarity of the rectifier diodes.
Simply run a circuit from the ignition power, through a switch (call it ALT 2, or something), through a warning light globe IN SERIES to the the alternator exciter terminal.
When you turn on the Alt2 switch, the 2nd alternator will power up and put current into the battery, if it isn't charging the warning globe will light up. If you turn off the Alt2 switch it just sits quietly until needed.
Bear in mind you can also use a 200amp alternator for stick welding.
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