What you're essentially asking for is two simultaneous outputs from OBS encoding different scenes, which OBS does not support. And even if OBS did support it, YouTube doesn't support ingesting two streams simultaneously, let alone PiP compositing. I don't know of any service that does that.
The closest you can get is to have 2 instances of OBS running, one streaming each video feed to different YouTube channels, so that users can open two browser windows and watch them at the same time. But that sounds like a pretty big hassle.