Yeah, if everybody is both their Meister and their weapon, then the weapons aren't stuck being bored. Since weapons don't exactly do much once they've transformed.
The bonds you make between meister and weapon are pretty essential to me.
These are both valid points. Personally I'd go with each player controlling both their own Meister/Weapon pair; you'd still be able to write the bond between the two characters yourself, though I can see how this wouldn't be as fun as working with someone else, and it eliminates the risk of someone dropping out and leaving the other person without a partner. Which I think is way more important.
In either case, I think making it a rule that each person has to write both a Meister and a Weapon would be good. Even if we decide to have people partner their characters with other people, it guarantees that we'll have an equal number of Weapons and Meisters.