When I first started dual-boxing I used a G15 keyobard. I had 9 keys for controlling my Pally and 9 keys for my Mages (and priest, but that aside). I used a toolbar mod such as Bartender / Trinity to graphically represent they layout of the G15 on screen. In the top 9 keys I had the Pally spells and they worked as any spell in a button would. But for the Mages I simply created blank macros and chose the icon of the spell I wanted to use. So it was a visual representation even though clicking the toolbar did nothing it allowed me to memorise the spell / key locations. After I got used to spamming the same keys 100000 times a day it started getting familiar and I eventually removed the "macros" from the toolbars.

Regards of the use of a G15 keybaord you can apply the visual trick to standard keyboard layout. Eg if you slaves / alts use modifiers such as Ctrl Shift or Alt and use the standard number keys 1 through = you can create a visual bar with all the spells. Sure clicking the spell buttons won't do anything because there is no spell in the macro, but it allows you to "see" and memorise where the spells are when you press a key.

Once you've done it for a few weeks you'll get used to it. Its really simple after a while.