You can probably use a single transistor, a resistor, and a diode to interface a 2 volt signal to the relay, but for specific help, you need to tell us the signal levels corresponding the the relay being energized and not being energized. If possible, post a schematic of your 2 volt circuit. We also need to know either the resistance of the relay coil or the current rating of the relay coil.
You can add a 10 volt battery in series. Be sure to take proper consideration of the two voltages when connecting them or else you might end up with 8 volts instead of 12.
Also see mcgyvr's reply.