Incorporating sub into my 2 channel / HT system

Iv'e been reading many blogs but still can't find exact answer to my question. I have a Marantz SR-18 receiver that I use for my HT. This only runs my center channel and sub......I don't use surround speakers as it's only a 3.1 setup.

I use a Cary CAI-1 integrated for my 2 channel listening. The Cary is also used to run my L/R mains during HT via cinema bypass loop. My mains are JM Lab Electra 926's my sub is a Velodyne SPL-800R which is currently run off my Marantz using the LFE. I want to augment my 926's during 2 channel listening as well. How do I connect my sub so that it performs double duty weather 2 channel listening or HT. Also, I want to add another SPL-800 to augment both L and R sides. I am a babe-in-the-woods when it comes to this stuff. How do I tie everything together? which in/out do I use? Do I need Y-splitters? Help!

I find SVS Micro 3000 allows current-time listening through my phone app, and through a Bluesound Node, you can also adjust ALL sub crossover settings,sound settings, etc. ,etc. combining these two jewels in any set up. 

Some of the REL subs allow two separate inputs. Separate level controls for HT and 2 channel. 

This might be a little nutty, but if you’ve got the budget and the space you could buy another sub and hook one up for HT and the other for stereo.  The added benefit is you have the ability to optimize each sub for its specific purpose.  You can get an SVS SB1000 Pro that has a small footprint yet still gets down to an honest 20Hz (-3dB), and it also comes with integration software you can adjust from your chair that is really convenient and helpful and all this for $599 with a 45-day, risk-free trial and free shipping both ways.  Just a different thought outta left field FWIW, and best of luck.