You cannot have duplicate DN's in the MiVoice Business, regardless of tenanting or interconnect restrictions. Even though tenants may have interconnect restrictions to prevent them dialing each other, it is still one system and it will only recognize one x100 or x1004 or whatever, they cannot be duplicated. Period. That's my final answer (I have been down this road as a sales person sold a system like this once, and it just can't happen).
The most common way of doing this is have a leading digit... for example, tenant one will all be 1xxx and tenant 2 have 2xxx, 3 will be 3xxx, etc. This allows them to keep their original 100 extension for example, just not it will be 1100 or 3100 or whatever.