Dual coil subwoofers are usually made for car audio, where 2 ohms is sometimes used in order to get more power out of an otherwise conventional amplifier (most car power amplifiers now use voltage inverters to get around this).

The amplifier should deliver more power at lower impedances. I would not under any circumstances attach a 2 ohm load to any home theater amplifier.

Whether the speaker will be louder - why don't you hook it up and see?