Based on his comment of "totally different RF hardware team (nothing is common, pretty much)" I'd assume not much can be shared, though I am curious too. Beyond just designing the phone, you also have the overhead of manufacturing two separate mainboards (based on photos showing the radio/SIM bits on the same PCB as the flash/processor), sourcing the different parts, additional logistics or shipping more SKUs, and so on.

I'd actually be interested in seeing a teardown of one of the Chinese iPhones (the ones lacking WiFi) to see how Apple currently handles that. With those phones it's just a matter of taking something out.

The iPad would likely be easier to go CDMA first, with the modem being a separate PCB that plugs into the mainboard. And with no voice, simultaneous voice/data issues go away.