Adding 2nd Int. amp to 2-ch. system


I'm not sure how to SAFELY add a second Denon integrated amp to my 2-channel system to be a power amp... it is an older version of the same model and only has pre-out (no main-in) terminals. I know someone who has done the following when adding one to just drive the front speakers in a HT setup, bypassing their HT receiver. So, can I do the same :

Connect the interconnects from the pre-outs of Amp1 to any input (apart from turntable, eg.aux.) on Amp2, selecting this same input for playback? Will I need to keep Amp2's volume set to anything in particular, like '12 o'clock' and why?

Is anyone doing this too and heard sonic improvements?

Thanks for any help,