What Nimo suggested is another useful way of doing it if you have a couple of amps - you can split the signal and go to two amps and try that. Again, you have to watch out for phasing issues but you can get a bigger sound by close miking the two amps and putting one L and one R.
Incidentally, if you ever want to freak non-audio people out, put a signal on two tracks, centre pan, reverse the polarity of one and have one volume at minimum and the other at 0, then slowly move the min track's fader up until the sound cancels out completely... almost as much fun as freaking them out with automated faders "(OH MY GOD - IT SHOULDN'T BE DOING THAT!!! It's possessed!)
And I don't think 1+1 = 2 in sound - it's more than 1 but you don't get twice the volume by adding an additional copied track - I know this was only being used as an example, however..
Cheers