Biamping uses 2 channels of amplification on 1 speaker. This is done most easily 'passively', in which one uses the 2 sets of frequency-divided input terminals on the speaker and drives each of those with a separate channel of amplification and speakercable. This continues to use the crossover in the speaker along with its frequency tailoring, etc. that the speaker's designer labored over to get right*. You could consider passive biamping as a further evolution of biwiring.
Simply drive each of the amp's channels with one signal cable from the preamp going thru an inexpensive 1-to-2 jumper available at RadShak. Since you'll be using identical channels of amplification, no gain (volume) controls are necessary. This does require 4 speakercables instead of 2, but I think it'll be worth it. BTW, if you decide to do this, a 125WPC B&K just might be powerful enough since you'll have 2x125-into-8 available for each channel.
* This point is ignored by those who use 'active' biamping, wherein a line-level crossover is used before each channel of amplification and where each amp channel drives directly each driver. This removes any sonic character, both good and bad, of the speaker's crossover. Of course, some crossovers/speakers have no frequency tailoring to ignore. I know that there's frequency tailoring in my LFT-12, so probably it's used in your 8s too.