[seg9585]

I'm not an electrical engineer, but I would imagine that the higher-voltage battery would supply back-current to the lower-voltage battery, thus charging the lower voltage battery until both are at equilibrium. During this process a bit of charge would be lost due to the battery's internal resistance, but this would be a converging process, not a "domino effect".

Batteries are setup in parallel all the time. I am an RC plane hobbyist and it's done to increase the charge capacity to get a longer flight time.