In 30 years I have only done this once. It was a Convention Center and a Hotel, Customer 0 and 1. What is the reason you want to do this? You also need to understand how it will operate. You share some resources but not all. Stations and Trunks are unique. You can't even call between Customer 0 and 1. Resources like Conference, TDS, DTR's are shared between customer's. When I got it all done and completed, guess what, they wanted to call between the customer's 4 digit. That would have required removing all the programming and making them all one customer. So we built a PRI between the 2 customers and used CDP to dial between customer's. What a mess.