I gave this a wee try with my paladin and 4 of my shamans. One of the shamans already had fishing at 330, but the others in that team were all at zero. I used the opportunity to level cooking as well.

It was actually far easier than I thought. As long as you make sure that all toons are standing on each other and have the same camera angle it's fine. One thing I noticed however is that if toons are of different size, that can cause some issues with some casts due to the different perspective. My team is one male belf, 3 male orcs and a female orc. The belf and female orc being slightly shorter than the male orcs occassionally missed the click. I tried to manually compensate by adjusting their screen....but on the whole, it worked out nicely.