A battery has a characteristic voltage. Voltage is a kind of potential energy; it's the existence of voltage across a circuit that causes current to flow, and current flow is of course what does work.
The voltage of a battery is a function of its charge. The more discharged a battery is, the lower its voltage.
If you hook up two voltage sources of the same voltage, you get twice the "push," so twice as much current flows, and you can do twice as much work.
But if you hook up one (relatively) high voltage source and one (relatively) low voltage source, what ends up happening is that current flows from the higher-voltage battery into the lower-voltage battery. This means less effective current is flowing through the circuit, meaning at best you're just wasting energy. At worst, the lower-voltage battery puts so much resistance on the circuit that you don't get enough current flow to do what the machine in question was meant to do, so it's effectively like having two dead batteries.
