Thanks for the quick replies.
berkeman: What I was trying to say is that when the dead car is being started, the starter resistance will be even smaller than the internal resistance of the dead battery so the current should flow from good battery to the (bad) engine instead of into the dead...
So I have a good car battery at around 12 V and a bad car battery discharged to 6 V. I hook up the batteries (sitting in the cars) with jumper cables. Now, before I turn the ignition switch on the dead battery car, there is effectively a circuit between the the good battery and the bad...