In Wakfu's current state, anything a multi-boxer can do, an organized team of 6 different people can do just as well. This is why i think multi-boxing is not p2w.
If we go with your suggestion, the multi-boxer is getting a significant exp advantage over the other 6 person team.
I always enjoy the company of others, therefore my point of view is towards those that don't. They should be allowed to have option to play with as many characters as they please, but they should not be forced to go through lower level content. The multi-boxer should be getting a significant exp advantage as he already went through the lower level content, therefore, he/she should not be opted to go through it.
The 6 person team, are first time players aside from the lv 140 in your example. They haven't played the content. To them, a lv 40 area is brand new, as for the lv 140, the lv 40 area is not new and earns nothing from the area.