Considering that...A.) Magic is expanding faster than eve...

[Ephraim]

The card type you're looking for is 'Enchant World' Centroles, and in that matter, you are correct. Only one is allowed on the table at a time. The idea still doesn't work for the Moxen, though, since each new Mox played by either player would destroy any other Moxen controlled by either player. It's just a cumbersome idea. WotC decided it didn't work well for Enchantments (ie: they're not doing it anymore) and it probably wouldn't work any better for artifacts.