@johns21 I recently went through this and think I've nailed it. Of course the best option is to have 2 rooms but most of us don't have that luxury.
To put them in the same room, here is my strategy: build your 2 channel system exactly like you like it. with a couple of caveats. your amp needs to have an HT bypass or just 2 inputs. the way I do it is I use an integrated amp with the volume turned all the way up. So it is becomes an amp with multiple inputs. I run my preamp into one of them and this is my 2 channel system. Note that most volume controls for integrated amps have a contact at full volume that fully bypasses the potentiometer or resistive ladder, whichever you have.
For my HT system, I run all the surrounds and center from the HT receiver but for the fronts I run a pre-out signal from the HT receiver to the 2 channel receiver (different input from the 2 channel preamp).
Set up levels in the HT amp just like you normally would. Now to go back and forth between HT and 2 channel you just select an input.
This is basicly HT bypass and if you have an integrated with HT bypass it works the same. A lot of people think that ht bypass is a compromise and puts extra circuits into the signal path for your 2 channel system. You can see that it doesn't.
Jerry