Nothing more than if both cars had the same terminal grounded to the chassis. The difference in potential exists only between the terminal posts of the individual battery. If the batteries aren’t interconnected, there is no difference in potential.
The actual current flow, I believe,is from the negative post to the positive post. However, it makes no difference as to which terminal the engineers design to be the “ground” terminal. I bought a 1948 Dodge and the son of the owner had installed the battery backward. The ammeter read discharge when the car was running and would indicate a charge when the engine was off and the headlights were switched on. I discharged the battery and recharged it the correct way and then repolarized the generator by momentarily bridging the armature terminal of the regulator to the battery terminal. This car had a positive ground and the battery was installed backward. We didn’t have semiconductors in those days that would be ruined by a reverse flow of current. As I remember, the radio didn’t work when the battery was installed with the negative terminal grounded, but worked fine after I corrected the problem.
As I remember, Ford and Chrysler products had a 6 volt positive ground system through 1955. Some General Motors cars in the 1930’s may have had a positve ground. GM then went to a 6 volt negative ground and phased in 12 volt systems beginning in 1953 with the Cadillac, Buick Super, and Oldsmobile. The Buick Special adopted the 12 volt system in 1954 and all GM cars became 12 volt in 1955. Tbe Imperial introduced by Chrysler in 1955 had a 12 volt negative ground system. In 1956 all U.S. cars went to the 12 volt negative ground system.