If what you want to do is possible you'll not have left and right channels...exactly. What you will most likely have is an echo chamber as the Arc and the analogue speakers will not play in sync. The Arc is not designed to allow any external left/right speakers.

I have a Panasonic DP-UB9000 which is comparable to your OPPO (IMO) which I have connected to my LG OLED TV.  The output is to my ArcUltra with Era 300’s and dual Sub 4’s that sound amazing (IMO) with Dolby Atmos or any other codec.

That’s a clever setup idea, and yes, technically the OPPO UDP-203 can output both HDMI and its 7.1 analog RCAs at the same time. The main catch is exactly what you’re thinking — sync. The analog outputs run directly from the DAC, while the HDMI audio going through your TV and ARC path will have a few extra milliseconds of processing delay. That can make your external towers slightly out of phase or cause a subtle echo effect depending on your listening distance and equipment latency. You can sometimes correct it with speaker delay settings on your receiver or the OPPO’s speaker configuration menu, but it might take a bit of tweaking. If your goal is just to get higher-quality front-channel sound, it’ll work fine once dialed in — just don’t be surprised if you have to nudge the timing by a few milliseconds to get everything locked in cleanly.

Thanks but let us say for example I am watching a movie on my Oppo with no HDMI connection to my Arc setup. I will hear a left and right only audio signal through my towers as they will be connected directly to my Oppo.which is also connected to my TV via HDMI. NOW I plug the ARC back in… do you see my point? It should in theory find my Sonos setup playing 7.1 audio as it has always done but now I also have added dedicated  left and right front audio channel speakers added to the mix. But I agree. the on thing IMHO that will be the biggest hurdle will be syncing the RCA audio outs with the OPPO and the Sonos Audio as it has to pass through TV first? 

Cheers and Thanks

My thoughts exactly. and I’ll tell you what started this journey. I moved my rather expensive main listening room towers up to my Theatre room as I have new ones and was really hoping there was a way to utilize them in both a Home theatre environment as well as a dedicated 2 channel critical listening environment without having to start over and buying all new gear. I Can use the OPPO to satisfy MOST of my theatre rooms listening needs as I will probably use it entirely as a SACD and Blu Ray multi channel audio source as well as ALL 4K and I think streaming sources. Most other critical listening outside of this will be on my main system downstairs. Cheers

There will be lip sync issues that may or may not be significant for you.