You can include multiple mass instances in a project. To eliminate overlap, use the Join Geometry tool to join a mass instance to other mass instances.
As a result, their gross volume and gross floor area values are adjusted accordingly.

Geometry panel
Join drop-down
The overlapping form of the first mass cuts into the second mass. The mass floors of the second mass are adjusted to report accurately gross floor area in a mass schedule.
If you move the joined mass instances, their properties are updated. If you move the mass instances so that they no longer intersect, a warning message displays. You can unjoin them using the Unjoin Geometry tool.