"...least national DFU code says..."

You have to be able to get rid of all the water that can enter.
As simple as that.

In the plumbing industry the abbreviation "DFU" stands for "Drainage Fixture Units" and it is the official unit of measure for determining the size of all DWV (Drain, Waste & Vent" lines.

Although the actual numeric value may vary from one code to another, the end result is always the same.

In your code there is a table that lists every type of fixture which may be connected to the drainage system and it assigns a specific numeric value to that fixture.

By example, under the IRC(International Residential Code) a shower is assigned a value of 2dfu's, when the shower enclosure has more than one shower head we must assign the value to each shower head that may be used simultaneously. According to your list you have a total of 6 shower heads so the combined DFU load for your shower stall would be 6x2-12dfu's.

Once we have determined the total DFU load we then consult another table that lists the maximum number of DFU's which may be conveyed by the different sizes of pipe.

By example, code generally calls for a 2" drain for a shower however, under the IRC a horizontal 2" vented branch line would be limited to a maximum of 6dfu's and a horizontal 3" line is limited to a maximum of 20dfu's, therefore, just as your inspector has already informed you, your options would be two install two 2" drains for a combined total of 12dfu's or you could install one 3" drain that can handle up to 20dfu's.

The only other option is to install diverter valves that would limit the number of shower heads that can be used at any given time to insure the dfu load does not exceed 6dfu's.

In my jurisdiction they would not allow installing the diverter to limit the calculation because there is no way to insure the diverter would not be removed at some future date, We are required to provide drainage equal to or greater than the combined load of all shower heads. regardless of how the valves are configured.

Under both the IRC & UPC we gave another table that lists the maximum number of dfu's permitted on the pipe for the building "Main Drain" and "House sewer".

Under the UPC we have a 4th table that is used for sizing vents.

Your inspector is correct in that your 12.9 DFUs are too many for a 2 inch drain, but only marginally so.

Under the IRC plumbing provisions, a 2" shower drain can only carry a maximum of 12.3 DFUs....so you would have no choice but to install a 3" drain...

Two 2" drains 'might' work but only if designed and approved by an engineer and only if your downstream drains could still handle the flow.

Your inspector cannot approve a 2nd 2" drain without a design professional signing off on the plan, and that translates for you into $$$ spent hiring an engineer.

Either reduce your DFUs by eliminating some of the shower heads, or go with the 3" drain all the way to your building sewer.

I don't know where Manhattan is getting 12.3dfu's for a 2" line,

On IRC table 3005.4.1 for branches and stacks a 2" line is permitted a maximum of 6dfu on a horizontal run and 10dfu's on a vertical run.

The maximum flow in any size of pipe can be calculated without going to the DFU tables.
Make the total match the carrying capacity of a two inch drain.

What distance is between the shower drain p trap and the next bigger pipe? Downstream.
What distance is there until venting?

With fixtures that spray less than 2.5GPM each, and body sprays less than 1.8GPM each, you might fit a lot of them onto a 2" drain.

-david