Any chance that the Starter is insulated to some degree from the Engine? A poorly grounded Starter, due to excessive paint or powder coat for example, could be the issue and may explain why it takes the Current from 2 Batteries to start the Engine.
The simplest test that you can perform is to disconnect both Battery Cables from the Battery as well as the Positive Cable from the Starter.
With a GOOD set of Jumper Cables, attach the Red Lead to the Starters Hot Post and the Black Lead to the Starter Housing; a Mounting Bolt should do nicely.
Now attach the other ends of the Jumper Cable to the Battery; first the Black Lead to Negative Terminal and then touch the Red Lead to the Positive Battery Terminal; the Starter should function normally. Keep in mind that this directly connects the Battery to the Starter without any Fusible Links. If still nothing, then it has to be the Starter or Battery.
If the Starter does turn normally however, then make sure that the Starter is not insulated in some way and also check that the Negative Battery Cable End is making good contact with the Block. Or if the Negative Cable is not attached to the Block, maybe to a Bracket for example, make sure that wherever it's attached as well as any subsequent connections that may be part of the Ground Path are also not insulated from good metal-to-metal contact.
Hope this helps and all the best.
Ray