How Do You Calculate Watts Per Channel?

If you are bi-amping with 2x200W (@8 Ohms) amps then yes you have 400WPC (@8 Ohms).

As Al said above, the instantaneous power is a function of the impedance of the speaker at a given frequency as well as the power supply of the amplifier.

Utilizing Ohms law, an "ideal" amplifier will double it's power as the impedance is halved.

200W @ 8 Ohms
400W @ 4 Ohms
800W @ 2 Ohms

etc.

I was not trying to state what Wattage was being consumed by the speakers at any given time, merely the theoretical power available which is how I read the OP's question.

I think most people think of this question in terms of a perfect 8Ohm load and not the various impedance swings which we know is reality.

I stand by my assertion that there is 400W on tap... should it be required. :)

In your first scenario the full 400W are being SHARED among all the drivers.

For sake of simplicity lets say there are 2 drivers, and assume that they have exactly the same impedance and efficiency. Therefore the two drivers will perfectly split the 400W, 200W per driver.

This is no different than individual drivers connected to a single 200W amp each.