I have done the same thing. The drain on both is equal IE: 50% coming from each one, your difference is that using up say 20% of the charge is not equal due to battery capacity. So 20% of 5400mAh is a larger figure than 20% of 2300mAh. The smaller battery will die first. At that point I don't think the circuitry in the DVD battery allows for current to flow back in through the output jack so you shouldn't lose any battery life to the dead battery trying to pull current.