Any source connected to your TV that is currently playing video, will have whatever interface it offers available on screen.
Optical audio is just a different way to move audio from a source to a destination. The problem with many modern products is that they don't have a digital audio optical output on them anymore. This includes Roku products. So, if you want to get the audio from the Roku to the A/V receiver, bypassing the TV, or going to the receiver through optical and the TV through HDMI you will need to test things out. Since your setup is unusual with the request for two A/V receivers, it makes things more interesting.
The second Roku, connected to the receiver directly, is a normal setup. We won't worry about that one at all. It doesn't really matter that much because that one will work just fine.
But, the one connected to the TV will depend on the TV itself and how it handles the ARC and the optical audio outputs and whether they can be simultaneous. They very well may be available at the same time, in which case the receiver with ARC and the receiver connected via optical audio would both be receiving audio from the TV at the same time, depending on what source was playing on the TV.
Another option is to use a HDMI to HDMI+Optical audio extractor.
They have 1080p models for about $20, but this one will properly work with 4K sources (it says)...
https://www.amazon.com/gofanco-Prophecy-Audio-Extractor-Repeater/dp/B07N11424X/ref=sr_1_1_sspa?keywords=hdmi+audio+extractor&qid=1551967582&s=gateway&sr=8-1-spons&psc=1
So, you get your HDMI video up to the TV and you get a digital optical audio output as well to connect as you want to. Audio will not be in HD audio formats, but will be a good 5.1 mix.