Unscrew the binding posts from the rear of the left main output speaker, remove the caps and lift off the pair of metal rods that join the upper and lower pair of connectors. They will appear as a pair of flat metal plates, around 10 cm long, with a hole drilled in each end. One hole slots over one binding post in each pair, joining the the lower positive post to the upper one, and the lower negative post to the upper one. Place these bars somewhere safe, in case you wish to return to a single-amp setup in the future.
Designate one of your stereo output amplifiers to be the "Bass" amp. Ensure it is powered off, and turn all the volume and gain controls to zero. Connect the left output to the lower pair of connectors on the left speaker. Connect the red post to the red post, and the black to the black. Repeat with the right speaker.
Connect the second amplifier to the remaining posts on the speakers using the same pattern as the previous one. Connect red to red and black to black from the amplifier output to the uppermost pair of connectors on the back of the speaker.
Power on both amplifiers and gradually increase the volume on each until it is at an appropriate volume.