Having multiple systems that do pretty much the same thing doesn't make a lot of sense to me. Especially if they're in the same room. So, I approached this question slightly differently. I started with a very good (IMO) two channel system to which I added HT/surround capabilities. Whatever I watch or listen to is played back in the mode for which it was recorded.
All of my two channel sources are connected to a two-channel preamp. Those two-channel sources are played back using my main (front) speakers with dual subwoofers. All of my HT/surround sources are connected to my pre/pro. The surround and CC speakers and amplifiers are connected to the pre/pro.
For two channel music, only my preamp, front amplifiers and speakers are used. My HT pre/pro and associated amplifiers aren't powered up. This way I have a dedicated two channel system in a dedicated acoustically treated room.
For HT/surround I place my preamp in bypass mode. I then turn on my pre/pro and the surround and CC amplifiers. Then my pre/pro controls the volume of all the speakers.
One of the more interesting things I discovered using my gear the way I do is, the auto equalization utility in my pre/pro sets the equalization within 1dB of what I measured and set using REW and a DSP. Consequently, the auto equalization on the pre/pro is bypassed for the front speakers. However, it is used for the surround speakers and level balancing. Flat in room response means the same thing whether it's for HT or two channel.
So, I have a dedicated two channel system that becomes a 5.1 system by flicking a couple of switches. Therefore, there is no compromising of my two channel sound.