The hard part of all this is playing 5 WoW characters at the same time. I'm still practicing the particulars of it but I will take a moment to go over the general playstyle.
My plans are to actually level 5 different classes at the same time unlike the traditional multi-boxer who usually plays 5 of the same character. In order to do this though I will need to be able to have all 5 characters be doing different things at the same time. The trick is in-game macros and keyboard broadcasting.
First, setup keyboard broadcasting so when you press a key it goes to all 5 computers. Second, setup a macro on one character, that assists the main character and then casts a spell. Put this on action slot 1. Then create a similar macro on another character that also does /assist followed by a different spell and put that on action slot 2. Now, notice that when you press 1 your first character does whatever you bound to 1 and when you press 2 your second character does whatever you bound to do. So you control your main character and use him to target things. Once something is targetted you would press 1 if you want your first character to do his thing and 2 if you want your second character to do his thing. So now, you are able to focus your attention on the primary character while still giving commands to the other 4 characters.
There are some types of macros where you do want all 5 characters doing the same, or similar things at the same time. For example, if you are playing a mage, warlock and priest at the same time you may want them all to cast their core "spamming" ability at the same time (shadow bolt, frost bolt, smite). So you could create a macro as listed above and put it on action slot 1 for all 3 characters. Now when you press 1 all three characters will cast their respective spells. Another very useful macro that you will want them all to do is /follow the main character. Since you will mostly be directly controlling the movement of one character you will likely want all of the others to follow you around so you can just press one hotkey and get them all locked back on to you.
Things quickly get tricky though. Not only do you run out of action buttons and keyboard space very fast you also have problems with creature and character facing and positioning. If you just have your 4 characters follow your main one into melee the 4 in the back will never swing at the creep because he will be too far away. If you try to run past the creep and hit him in the back the creep will often move slightly to compensate, throwing off the other 4 characters. If the creep does AoE you may not want your characters to hang out in melee range.
One solution to the positioning problem is to manually control them all. To do this you simply have to move the mouse over to the appropriate monitor and use the mouse to position the character. The obvious problem with this is that while you are doing that your other 4 characters are just standing there or auto-attacking. Another option is to bind auto-follow keys for your "tank" separate from the auto-follow keys for your damage dealers (have your primary be a healer or something). Then you would have the tank charge in or pull while the damage dealers follow the main character around into better positioning. If the main character is a healer they can afford to take quick breaks to reposition the other characters without hurting the groups performance very much. At first I planned on going with the first option but I'm seriously considering the second option after having played around with it all a bit.
My quest to level 5 World of Warcraft characters at the same time.