Think it depends on what you're trying to achieve. Xbox has posted videos on You Tube of five shammies doing Heroic Mana Halls (and great fun it looked too), the above post also has a video of one person doing a heroic. TBH if you have the skills and patience I doubt its that hard.

However I got the feeling you want to dual box a tank and a priest (or healer) with the remaining three characters controlled by other people. That would be IMO much harder. For two reasons: 1)Unless they know exactly what you're doing and support you in it, they won't understand if something goes wrong in a script and you fail to heal or lose agro. 2)Whilst dual boxing is great fun and great things can be achieved whilst doing it playing two characters as well as you can play one is IMO virtually impossible. Especially if those two characters are totally dissimilar.

Bottom line is for normal runs three people are relying on you to do your job just right, if you feel capable of that great.