L&R front pre-outs from the HT receiver, connected via RCA interconnect cables into the HT bypass inputs of the pre-amp or integrated amp. Once you have this external amplification connected to the receiver, re-run the receiver's setup routine to account for the different amplification specifications of this external amplification.
As you can see, the "add-on" HT does not affect your 2-channel system in any way, and the HT receiver does not even need to be turned-on to play 2-channel content (as long as you have your 2-channel sources connected directly to your pre-amp/integrated amp).
And when you want to play multi-channel/HT content, turn on the HT receiver, select the HT bypass input on the pre-amp/integrated amp, and now the amplification and speakers in your 2-channel rig become the front L&R amps/speakers for your HT/multi-channel rig.
I've done it several times, with different systems I own, since my 2-channel systems and HT systems are in the same room. Great, easy way to share your main speakers between the two systems, and not have any sound quality degradation from the (typically) compromised HT receiver.
