connecting mono amps to pre amp

What mb1audio02 said is correct, although given your response what he said may not have been worded as clearly as it might have been. You would connect an interconnect between the right channel input of the right channel amp and the right channel "preamp" jack of the 6100. You would connect an interconnect between the right channel input of the left channel amp and the left channel "preamp" jack of the 6100. The left channel input of each amp would be left unconnected. You would set the rear panel switch on both amps to mono, as you indicated.

Regards,
-- Al
 

In addition to Al’s comment and hopefully he will verify or correct if I am not.

After looking at the MC2120 manual here,

http://www.berners.ch/McIntosh/Downloads/MC2120_own.pdf

You actually connect the speaker to the left channel output of the 2120 for mono operation. Negative of the speaker to the "COM" terminal on the amp and positive of the speaker to the appropriate impedance number tap with a jumper from that tap to the same impedance number tap on the right channel output.

so If I under stand this correctly the left out of the 6100 will go to the right on one of the 2120's and the right out will go to the right of the other 2120. then you would bridge the common/negative on the speaker terminals. In addition on the front of the 6100 the mode position would be in the MONO setting that states (MONO) L + R   .   Is that correct? and MANY thanks!

Al is right. I thought you had 2 mono amps. It never occured to me that you wanted to bridge 2 stereo amps.