Just check www.space-tech-lab.com, their web site have the 10 free tips section which describe how to calibrate a bi-amping system.
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It's easy to match gains of the two amplifiers. Just feed them both the same signal and put your voltmeter between the two high signal (red) terminals. Adjust gain so that the meter reads zero.
However this is a complete waste of time because your HI and LO speaker drivers probably have different efficiency. So what you need to do is get a sound pressure level meter and adjust the gains of the amplifiers so that each one, when playing the same signal, produces the same loudness.
And when you are all finished with this, adjust the gain so that the music sounds good to you.