Long before you damage the primary pack, it will have discharged to a low enough voltage that the backup will pick up the load. In the extreme case, if you are running the regulators at 6.0 volts, you can see the primary will eventually begin to drop off below 6 volts when loaded and the secondary will clearly pick up the load. At that point there is no damage to the primary. However, if you are running that imbalanced and for that long, you will have lost any redundancy.

When running an imbalanced system, it is important not to fly to the point that you have depleted the primary past the point of supporting one more flight. Of course if you use the motor batteries as your primary, this is not a problem.

John