OK, it started using two batteries. Have you changed both battery cables? I know you stated earlier you were going to. That extra battery might have given enough *push* to get the juice through an internally corroded cable.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.