Recently, I received the following question:
"Have you ever seen a computer with 2 mice (left and right) connected at the same time? I am sure it is possible because I saw it but I don't remember where."
As it turned out, she was looking into this seemingly odd idea for a good reason. She wanted to take some of the strain off her right hand. Many people, including me, suffer the effects of a lot of mousing around. Although I haven't seen exactly seen two identical mice hooked up to one computer, it is possible. You could have a PS2 mouse and a serial (or USB) mouse connected at the same time, since they use two different ports on the back of the machine.
In fact, this arrangement is somewhat similar to the way my computer is set up. I have a pen tablet that I use with my left hand (I'm left handed). But I also have a mouse connected that I use with my right hand. (When I started using computers, there were no left-handed mice, so now I can only use a traditional mouse with my right hand.)
As I got into graphic design and pointing devices improved, I realized that it was completely ridiculous that I was actually drawing with my "wrong" hand. I was a left-handed person drawing artwork with my right hand using a mouse. So, I invested in an early pen tablet and never looked back.
Later, we got another pen tablet for a different reason. My husband (a programmer) was starting to have trouble with carpal tunnel-type symptoms from using a mouse. He'd programmed character-based systems for years with no problems. But as soon as he started programming for Windows, he needed a mouse to design all those Windows graphical user interfaces. And his wrists and arms started hurting from all that mouse time. So we invested in a pen tablet for him and it made a huge difference. Every time he has to use a computer without a tablet, the mouse causes pain again.
Our pen tablets are made by Wacom (http://www.wacom.com/productinfo/index.cfm). The little Graphire 2 tablets don't take up much more space than a regular mouse pad. They cost about $100, which obviously is more than a standard mouse. However, it's a lot less expensive than any medical treatment for carpal tunnel syndrome, I can assure you.