I can't really offer any advice hardware wise, when I set this up I just bought two identical wireless keyboards (the cheapest I could find) and then used one of those keyboards with both receivers. I'm not sure about cross compatibility of those things.

Unless you can turn off a receiver, there's no way to stop certain keys from activating on both machines. For spells this shouldn't be a problem as you can either leave the actionbar on your alt blank and then use other bars or an addon to bind the macros to keys, or rebind the keys in the interface menu. It can however be a pain for things like movement (you'll need to unbind movement keys on your alt for whichever keys you use on your main to move) and also I found that even when i change the chat bindings so that my alt doesn't repeat everything my main says the alt has a happy little spasm with key shortcuts and I often look over at that screen to find a number of menus open and error messages. Ideally you'll want to unbind all of those key shortcuts too (e.g. 'm' for map, 'o' for options or whatever - can you tell i don't use key shortcuts?).

Macro wise I'll make a few comments, but I think macros are fairly personal things, so I'm not saying you're doing it wrong, just if it was me I might do it differently.

You have two macros to dispel magic from your main

Why not change:

Code:
MACRO 12 "Abolish" INV_Misc_QuestionMark 
/castsequence [modifier:ctrl , target=Self] reset=30/combat Abolish Disease, Dispel Magic; [target=Main] Abolish Disease 
END
to
Code:
MACRO 12 "Abolish" INV_Misc_QuestionMark 
/cast [mod:ctrl,mod:shift,target=player][mod:shift,target=main] Abolish Disease; [mod:ctrl,target=player][target=Main] Dispel Magic
END
Which would cast Abolish disease on you main if you hold down shift (or whichever modifier you like), your alt if you hold down shift and ctrl or Dispel Magic on your main if you don't hold down any modifiers or alt with ctrl held down (but not shift). One button for both cures and you don't have to cast abolish disease before dispelling your alt.

In macros like the following:

Code:
MACRO 7 "Seldom" Ability_Ambush 
/castsequence [modifier:ctrl, target=Self] Power Infusion; [target=Main] Dispel Magic 
END
I'm not sure why you are using /castsequence, rather than plain /cast
in the above exaple if you're holding down ctrl it will PI your alt, if you're not it will cast dispel magic on your main. There's not any need (that I can tell) for using castsequence.