Under Windows XP you can easily accomplish multiboxing of two accounts without any additional software.

Set wow to 800x600 and play on a widescreen monitor (1680x1050 gives plenty of room).
Set Windows for "Focus Follows Mouse - XMouse" using a registry hack (or use Windows PowerToys)
Setup your two classes.

I played rogue/priest like this for quite some time, you mouse over the priests window and click to heal, mouse back to the rogue and spam hotkeys for dps, back and forth and back and forth.

Is it OPTIMAL? Not in any way... and I can say that I've tried doing this with three characters and while possible, far from easy.