Anything is possible.

Like mentioned above 2 boxing is cake. If you wish to spend a bit of money ISboxer is 10$ for 3 months and is very user friendly.

You could manage a priest/mage combo no problem. With ISboxer and an addon like Healbot/Vuhdo you can heal a whole group from your mage no problem. You could even set up some macros to have your priest DPS- or just keep them as a pocket healer. Or even a group healer for randoms- wouldn't be too hard just getting use to the controls.

Multiboxing either 2 or even 10 is a bit of a learning curve. You have to get the hang of it but once you do it's easy.