How much of an increase in wattage does bi-amping create?

Bi-amping is using one amplifier to power the speakers' woofers & another amplifier to power the tweeters (or power the mid-range speakers & the tweeters).

You need to use an external 2-way crossover. The outputs from your pre-amplifier go into the crossover. The outputs from the crossover go into your amplifiers.

To answer the question at hand, if both amplifiers are the same wattage, there is no increase in wattage.

But if you are bi-amping your speakers, since the woofers use more wattage, the amplifier powering the woofers should be higher rated than the amplifier powering the tweeters.

If your speakers are not designed to be bi-amped, I would not bi-amp them.

JFYI

What Tim S described is called dual-amping, or multi-amping.

If you're not using a pre-amplifier, it's called bridging.

There are two ways to bridge.

1. You can bridge a single bridgeable amp by combining the left & right channels to make one mono channel.

2. You can bridge two bridgeable amps together to combine their power.

It's very important to make sure your amplifier is bridgeable, if you bridge an amplifier that is not bridgeable, you will damage the amplifier. If it is a bridgeable amp, the owner's manual will describe how to bridge the amp. If the owner's manual does not describe how to bridge the amp, it is not a bridgeable amp.

You WON'T need a crossover...Just because you hook up ONE set of speaker cables to the HIGH-END (tweeter) and another set to the LOW-END (woofer and mid-range) jacks on your speakers DOESN'T mean it BYPASSES the internal crossovers.

If you have a 100 watt amp going to the BASS and then another 100 watt amp going to the TREBLE it equals 200 watts...It's pretty simple...But your high-end input ONLY requires about HALF as much power at your low-end.

If you don't have TWO amps but you speakers have high and low-end inputs you can still BI-WIRE them...You just need TWO sets of speaker wire running to EACH speaker...OR you can buy speaker wire with FOUR SEPARATE conductors inside.

You hook up all FOUR at the speaker...And then at you AMP you combine the TWO POSITIVES and the TWO NEGATIVES...Plug the them both into the SAME jack on the amp...Just TWIST the pairs together turning 4 into 2.

That kind of wire costs about $3 a foot a Home Theater specialty store...I recommend going to LOWES or HOME DEPOT and purchasing "18/5 Thermostat Wire"...It's 20 cents a foot...It has FIVE, 18-gauge wires inside...I combine 2 pairs and then clip the 5th one.

The wire is SOLID CONDUCTOR for the best signal transfer...Also the 5 separate wires are TWISTED inside the ONE cable to resist ELECTRICAL INTERFERENCE...All of the MULTI-STRANDED, BRAIDED wire is GARBAGE...Don't waste your time with it.

Meaning adding an additional amplifier to your existing system through a pre amp hook-up? It would add what is stated on the added amp