Guessing that you are using Maximo Spatial then and ESRI.
Question 1: Are the Bus "truly" different Sites? Always a good question to answer completely first. Do they plan and execute work independently? Are their Assets managed independently? What about inventory.
If they are, then the best answer is probably to have BU Specific Map Servers on the ESRI Side, and be BU-specific in naming the GIS Objects.
Thanks for your time,
Scott J. Mahon, P.Eng.
Maximo Solution Architect