Thanks for the replies, guys. The second battery is in series, negative pole to "output side" of solenoid and positive to starter.
https://store.schnitzracing.com/24-v...iring-diagram/
https://www.youtube.com/watch?v=goO4VHemPbY
Neil, this is probably the answer!
A small battery would choke the current flow if paired with a large battery.