HT and 2 channel in same room? 1 or 2 systems?

I am not sure of your specific questions other then the title, and that is pretty vague. I can share with you my approach when faced with the same general question where I had one space and wanted to optimize several factors - appearance, cost, sound quality for 2 channel . . .

I bought a Benchmark DAC3 with HT bypass. It has worked great for me. The DAC serves as a pre-amp also allowing me to input a digital signal from my streamer and also the analog signal from my Onkyo AVR. With a press of a button on the remote, I can be only two channels (digital input from streamer) using 1 amp and 1 set of R/L speakers.  Or I can switch to the analog inputs of the DAC which connect to the front two pre-amp out channels of the AVR . There are also outputs on the DAC and AVR for low level outputs for my two subs. I use a paired benchmark amp. I am very happy with the flexibility and sound quality. I find the two channel audio to be excellent and it is very easy to switch to the AVR inputs for TV/Movies allowing me to use the same two channel amp and R/L speakers.  The improvement over using the DAC/AMP in my AVR was very noticeable.  

OP, a lot of great suggestions here, many requiring $$. I agree with:

6. - Pay someone with the specialized sound equipment to set up your speakers and room - it will ay of in the pure audio system as well.  

Either that or buy the equipment and DIY. Do this first  with what you already own and it is much easier to proceed with anything else. 

I have been using 2 systems in the same room for 20 years: This way each system can be optimally set up. To get the best HT performance, you need to have a wider placement of the main speakers which in most cases will adversely effect 2 ch performance. The 2ch setup does not provide the big expansive wall to wall sound because of width restraints. Another problem is there is no dedicated center channel speaker to match the LaScallas. This would most likely would cause the center speaker too not blend in and always be localized. The woofer problem is easy. Almost all RELs from the original company from the Richard E Lord days will have 2 dedicated inputs that allow high level and low level connections, each with dedicated vol controls. So you can use the bass management in your avr for HT and the ReL crossover, high level for 2ch. Not sure if the new ReL company run by John Hunter will do that. I have been selling 2ch and HT for 30 plus years so I hope this helps you. I use this set up in my own home and it works flawless.

@kota1 

Thank you for the suggestions, especially the upgrade for Audyssey.  I have had that Denon AVR for a couple years but have never plugged it in.  I guess I didn't realize that it would take me a year to finish my basement / theater room.  I have a JVC projector sitting in storage that I've never plugged in either.  Same goes for the Oppo 103d BD.  Hopefully all this works as planned when I finally get it hooked up in a few weeks.  Fingers crossed.

I think the OP had another concern about how to set up subs in a dual use situation which is my concern also.  It was said earlier that using a sub that has dual inputs would work for this use case.  In my case, I found REL and Sumiko to be appropriate for this use case for 2 channel as well as HT.  Not sure how well they will work for this but it is their intended use by design.  I am not a big bass head, but do require it in my music and movies as I can easily hear the difference that subs bring to the stage.  My goal will be subwoofer integration into 2 channel and ultra low for HT.  I'm not sure about the latter with the Sumiko S-10's but I intend to DIY some 18's or 21's to help out with that and hopefully pick up the lower 20hz or slightly lower if I can get there.  I have heard that on a concrete slab, getting below 20hz is wishful thinking for me.  I'll give it my best shot.  Thanks to everyone for the information that has been put out that helps me and sorry to the OP if I may have hijacked some of your thread.  I believe we are both searching for the same answer to the same question.  Good luck to you.

@speed121

I was struggling with the same issue.  I decided that I would set up a home theater system and then add a two channel system off of front pre-outs of the HT receiver.  I wasn't to do that because when listening to music I don't want all of the signal processing in the HT receiver.  Also there are those who are saying that a good two channel integrated amp should should much better than a HT Receiver.  With the right integrated amp you can use the HT Bypass option on the integrated amp to essentially tie it into the HT system.... and then when you want to listen to two channel music you can shut off the HT receiver and just use the Integrated amp.  I also plan on plugging my streamer into the Integrated amp so that the HT Receiver is completely out of the mix.