Any reason you don't use the X800M2 for video as well or are they on separate systems?
No, it's the same system. An Atmos 9.1.6 set up.
Two reasons:
The main reason is that many music discs, in particular Blu Ray audio and DVD-Audio require a monitor to get through menus, sometimes to set up audio preferences etc. or to make track selections, and I can't stand the bright light of a large monitor, it breaks my concentration. By analogy, a 55" monitor is like stage lights, the little 22" monitor is like a candle.
Second reason, I've had audio/video syncing problems (aka lip sync), so I'm experimenting with connecting the sources (in my case, the UHD player and Apple TV) directly to the monitor, with audio via eARC for (I hope) better communication between the TV and processor. In that case I am stuck with leaving a large monitor on even if I'm just listening to music. I can, of course, shut off the picture, but it still sucks up electricity, in comparison with the 22" (about 15 watts), and as mentioned above, I really hate the intensity of a 55" monitor to just get through some menus.
So yes, I could in theory just use the X800M2, but I much prefer keeping it as an audio player. In the U.S. an X800M2 plus an X700U at current prices from that monopolistic online retailer is about $600, and there are frequently "used like new" available at a fine discount. Of course, if I was not able to afford (or weren't so annoyed by the big screen with music), an X800M2 would do the whole thing, as you are pointing out. On the economics, if you listen to a lot of music, a large monitor will increase your electric bill.