watts per channel when biamping

Depends. What are you driving? The numbers you offer are only what the amps are supposedly capable of doing under test, not what you will get from them under normal conditions. Think about this: if you bi-amp, do you really think most tweeters can handle 100w, let alone 200?

When you bi-amp you are separating the speaker drivers, electronically speaking, and each divided portion of the speaker system will see only one amp. Your bass side will only have the max output of one amp and the mid/upper end will see only the output from the other amp. Ergo, even with two amps you will not be doubling the available power to either the high or the low end. If you are looking for more power, and you can afford to buy it, I would always opt for a higher power amp in stereo or mono. That way you don't have to deal with the cost of more cables, I/C's and XO, as well as potentially negative effects of introducing an outboard crossover/cables (if you don't use identical amps).

So i can't do a horizontal biamp without using a crossover if they are different brand amps? I am driving B&W Matrix 801. thanks

Yep - at a minimum you have to match the levels of the two amps output. Note in the amps spec's the input sensitivity level - if they are different, as is usually the case, you will be, in effect, creating an imbalance between the highs and lows if you don't use a crossover.

I biamp with completely different amps with no external crossover.
For bottom end I use Sunfire Symphonic Ref and for the top end I use VTL MB100.
VTL has sensitivity 30% larger than Sunfire but Sunfire has substantially(20%) larger gain than VTL.
I use for that purpose balanced line converter with adjustable levels thus taming the gain of Sunfire to reach tonal balance with VTL.
Ratings of Sunfire is 250W/8Ohms and 500W/4Ohms
Ratings of VTL is 120W/8 and 4Ohms.
Since I tame the gain of Sunfire by approximately 10%(taking onto consideration an input sencitivity difference),
I can assume that the output power of Sunfire is decreased accordingly(may not be exactly precise) so the total output power figures approximately 345W/8Ohms and 570W/4Ohms.

Please note that I wasn't willing to perform a precise calculations and adjusted the levels of amps by ear through playing my reference recordings with voice, piano, orchestra and instrumentals.

Sounds terrific!