Although the wording of the code itself is somewhat confusing, the interpretation provided in the Guide to 2016 California Green Building Standards: Residential is pretty clear. Quoting from that document:
"For example, if only one valve supplies a shower enclosure or shower area, the maximum water flow, regardless of the number of showerheads and other outlets, is 2.0 gpm at 80 psi. If two (or more) separate valves provide water to separate showerheads and other outlets, the maximum flow rate for each valve would be 2.0 gpm at 80 psi. If the operation of two or more showerheads and body sprays controlled by a single valve results in more than 2.0 gpm 80 psi total water flow, then only one showerhead may operate at one time with a maximum flow rate of not more than 2.0 gpm."
To get back to the original poster's questions:
Can you have two 2.0 gpm* showerheads connected to separate valves? Yes.
Can you have two 1.0 gpm showerheads connected to the same valve? Yes.
Can you use a diverter that allows more than 2.0 gpm from the same valve [Eric's scenario]? No.
* [For consistency I'm using 2.0 gpm, but note that the limit is 1.8 gpm as of July 2018.]
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Dual Showerhead code clarification
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