This is the same basic question I posed a while back and no one has had a definitive answer, and I have not been able to sleuth one out from the internet either.
So, I looked at the impedance graphs when you change the number of drivers in WinISD, I didn’t see the base impedance change as you’d expect it to such as when going in parallel or series. Either way you add the RMS up and the max up to get the composite of each. Now, if you are using the 4 ohm, you would most likely (and should in this case) be wiring in series. That means that for each driver, you are dividing the power by the # of drivers. 2 in series means that at 1 watt, each basically sees 1/2 wattage hence would only move approximately 1/2 the excursion it would at 1 watt each. But then again, you are doubling (at least) the piston area. So it seems that multiple driver scenarios in WinISD are not that easy. Hmmm….