it depends on the power requirments, if your using a 20 watt amp and maxing out the 20 watt amp.. bi-amping would make a HUGE difference as you would not max out the capabilities of a single amp. On the other hand using a 100 watt amp and then biamping with another 100 amp and then you would actually just use 20 watts or so.. would make a little difference
Biamping could also be advantageous when you want a "high" quality amp for your mid/hi drivers and a good high power amp forr your woofer....same scenario in a Bookshelf / Sub setup Class A or AB for the BS and Class D for the subs.
biamping using 2 identical amps is the way to go...why?, it is because identical amps will have the same gain, balancing becomes easier...
do not be afraid that you are only using the 100watt amp at 20 watt levels, you are not wasting power this way..
using a higher powered amp, say 200watter at lower levels simply means that you will have more dynamic range available this way, even if on the average you will just be listening at a 2 to 5 watt level in the home environment.
with a high powered amp, clipping is likewise avoided most of the time. remember that a 20 watt amp will always clipp earlier than a 200watt amp...