To test my question, I enabled SQM QOS and turned on a Twitch stream in Chrome and a game download in Steam. The Twitch stream would pause or stop when the Steam download was going. Then I lowered the download speed limit in SQM QOS to 50% of the internet connection's bandwidth, and even while the Steam download speed was limited so severely, the Twitch stream would still pause or stop. Is that expected behavior?
PS: SQM QOS and cake is amazing tech and there should be a dedicated forum for it somewhere. Probably here on openwrt.org because this is the software most people will be using when they use Cake SQM QOS.
There is https://github.com/tohojo/sqm-scripts where you could (and should) open issues, but there is really no forum over there, so here is the better place....
Well, sqm will default to per-flow fairness, but I assume that steam probably uses multiple concurrent flows while twitch probably only operates a single high-bandwidth flow. Once the number of concurrently active flows is larger enough that the "fair" share of bandwidth each flow will get is below what twitch needs you will see buffering/stuttering.
Cake allows per-internal-IP fairness which allows to share equally between different concurrently active computers, probably allowing concurrent twitch and steam, albeit not if both use the same IP address.
But you could just follow https://windowsloop.com/limit-steam-download-speed/ and throttle steam so that twitch will work even on the same machine...