Left Handed Mouse
I always use 2 mouses, since about 1994. The reason is that it lets me use either hand, whichever is convenient at the moment.
One problem is that if you use a 5-buttons mouse for left hand, usually they are symmetric design, so the button placement isn't optimal.
Here's a list of mouse designed for left-hand.
Here's a even better one.
See MMO Mouses