How do you calculate watts per channel?

No, when you vertically biamp, the power does not add together as some folks think. You will still effectively have only 100 watts per channel whether you bi-amp or tri-amp. To get more power you really need more powerful amp(s).

At best you will have no crosstalk between the right and left speakers and you may gain a little bit more headroom because one amp channel will be relieved of bass duty if you're using a stereo amp. If you're using mono amps the one on the midrange/tweeter section should have a tiny bit more headroom.

This is the simple answer as it's usually a bit more complex in reality, considering the varying impedance/load issues.

As you can see, there seems to be a lot of confusion on this topic, so let me explain it in more simple terms.

If your preamp sends a signal to play a midrange note at 500Hz, the amp that is hooked up to your midrange/tweeter section can only pull 100 watts out of a 100 watt amp. It cannot borrow ANY power from the amp hooked up to the bass section. Thus, you still cannot get more than 100 watts to either speaker section...

Effectively, you have individual 100 watt amps, each supplying 100 watts (not 200) to their respective speaker sections and power bands... Likewise the bass speakers cannot borrow any extra power from the midrange amp when it amplifies a strong bass note. It is limited to the 100 watt amp that it is actually hooked up to.

Some reading that may help a little.

http://sound.westhost.com/bi-amp.htm

Dave