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.