Agreed with above, having separate heal buttons is key I recommend using a Logitech G15 keyboard and setting aside a 6 block of buttons for healing 5 assigned to the various party slots, 1 assigned for group heals such as prayer of healing/chain heal, etc. I'd also work in a healing button or 2 as part of your main bar to heal the tank exclusively.

Also from my practical experience I'd segment your damage dealing affects so the same buttons arn't used for all DPS. I.E. for my setup I have the following. I.E. my shammy, hunter and warlock all use 2 buttons for dps cast sequence macros, however they don't all use the same 2 so I can effectively scale back damage from one toon or another based on threat so I don't pull aggro away from the tank.

It really depends on what classes your running. I run multiple toons as I already had a variety of differant classes at 70 when I started multiboxing, but if your leveling fresh characters running multiple of the same/similar classes is much easier...