Two mice, one left handed and another right handed. At the same time

This looks like a job for AutoHotkey! Rather than reinvent the wheel myself, user gwarble over at the AutoHotkey Forums has already created an application that does exactly what you want:

EitherMouse

• Multiple mice, individual settings...
• New features means EitherMouse isn't just for lefties anymore!
• Leave 2 mice on a PC and automatically swap buttons on the secondary mouse.
• This allows a user to leave a left-handed and a right-handed mouse always connected and ready to use, or...
• Useful for a multiple user or public workstation with both left and right handed users. The active mouse is detected and buttons are assigned accordingly and immediately.
• Individual settings for multiple mice besides swapping buttons, like cursor, speed, etc.
• New experimental MultiCursor mode keeps each mouse's location separate!
• Easily swap mouse buttons from system tray if only one mouse is desired.
• Tray icon points to active mouse.

I do exactly this to avoid repetitive strain on my wrists. It works in windows 7 if the mice are different brands and you install the manufacturer's software for one of them. Then you use the generic windows control panel to assign mouse settings, then use the manufacturer specific software to change settings for one mouse only. You will have two mice with different button configurations controlling one cursor. It works with very basic mice, too. For example, I did it with a Dell cheapo mouse and a low end Microsoft mouse, using the MS mouse software (probably coded for higher end mice, but works on all of them apparently).