The hardware is totally irrelevant. Mac, PC, whatever. It's the same hardware.

I've used Clonekeys on Mac OS, and you cannot change do-not-pass keys, which is real annoying. Mac OS also uses OpenGL, which is inferior to DirectX when running more than one client. Your framerate will drop TREMENDOUSLY because of software limitations, not hardware. Again, blame OpenGL.

Your main question should be "which operating system is okay to dual-box on?"

The answer is easy: Windows XP or Vista.

I five-boxed on a MacBook Pro (early 2008 model) until I reached the level where I needed to enter Shattrath City. Level 63/64-ish - It ran fine. Shattrath wasn't happening. At all. The key was running Windows XP via Boot Camp. Even with virtual software like Parallels or VMware Fusion, it ain't happenin'. Emulation is a big no-no with realtime rendered graphics!

Macbook? Hell no. Had one, running ONE client got 17fps in Shat. Terrible for gaming! (Early 2008 Model)
Macbook Pro? Sure. As long as you're running Windows. (DirectX)
PC Laptop with specs comparable to a regular Macbook? Hell no.
PC Laptop with specs comparable to a Macbook Pro? Sure.

The hardware is irrelevant as long as you're comparing apples-to-apples. (Specs)

If you're only running two clients, I'd say you'll be alright on a Macbook Pro in Windows. I did it with five clients.