It may not be a rule, but if someone were to do such a thing, they would have to cite canon evidence. If you can have multiple beats inside you at once, what's the point of the mazo? Even Naruto only held small portions of each Tailed Beast's chakra, he did not host all within his body at the same time. Heck I don't even think with SL supped up buffs anyone could make a seal that could hold that many biju.
What people could do however, is take small portions of each biju's chakra. I remember there use to be a rule saying not to tamper with the biju's chakra. So if that rule were put back into place, it would stop that. Also if that rule isn't in place, people could make Gold and Silver Brother equals by splitting biju chakra up. So there seems to be a few rules lost in translation. So we can open a thread for each and try to vote them back in, or we can go Athos route and just put them in, as most of these rules have long been in place, and may have been accidentally forgotten about.
So I guess discuss what we should do, then move on. Just trying to make progress and get these rules on the books or accept them as gone.