I'm not sure if you've found your answer already, but here are my thought based on reading others posts.

Pallys are generally a poor choice for multibox healing due to a good AoE heal. If you're only planning on running in a group of three, then it's likely not a problem. If you're planning on doing 5 mans where you control the healer, then it may be difficult compared to a shaman, druid, or priest.

Pallys are a favored tank choice for multiboxing, though many also use a DK (some use a druid, seems pretty rare to find a warrior tank)

If you're using one of the pallys to DPS instead of heal or tank, then it may be difficult. Multiboxing melee is generally considered more difficult than using ranged DPS due.

Some people try to stay away from multiboxing with a mage as it can be hard to maximize DPS due to unpredictable procs causing certain spells to be ideal. However, many do run with a mage in their mixed group, and some run with all mages, so I don't think it would be terribly difficult.