This was tested in Ballista on Diorama Abjalis (sp).

The multi-casting component of Meteor would not activate when we cast it on an opposing player. That is to say, If I start casting meteor and another person starts casting it, they're treated as seperate spells, doing the single-caster damage to the target, instead of combining to create a more powerful spell.

It is presumed the same thing will happen if it is cast on a charmed player, but I don't know for sure.

This may or may not be intended, but I figure i'd post here in the off chance it could be verified.