Currently a separate Calibration requires a separate TBC project Wayne.
Having said that if all your Base Locations are Surveyed In and correlate well with each other you do not need a different Site Calibration just because you have more than One Base Station, however they have to be cross correlated to each other accurately in order to do this (and I would recommend that for a large job requiring multiple Base setups).
The time wen you need multiple Site Cals is when the Geoid Model combined with a single Tilted Plane in the Site Cal still leaves high vertical residuals after the site calibration is computed, at which point breaking the site into 2 or more parts, each with its own site cal maybe required in order to get GPS Elevations to jive properly in each area.
When you have to break up the Site Cals, then you really need to treat each area as a separate project.
The Design Model can be computed as one, however for use in the field it will need to be broken up to match the areas of the different Site Calibrations. You dont actually have to break the Design, you could use it in several projects, just have a different Site Cal in each project. I would however want to place a boundary around the area where I would want the users to use a different Site Cal however to avoid working in the wrong area with the wrong site calibration.
When you do break up a project into Areas with different Site Cals, make sure that you have common points in each adjacent area along the dividing line between the areas, so that the Site Cals match along the common boundaries. You may want them to overlap a little also so that you get a good check on matchup between the adjacent areas.
Alan