Would it be feasible to do picture-in-picture in hardware? Digital

>>927271
If you can wait til march, then one of these should do: https://www.crowdsupply.com/numato-lab/opsis

If you can't, then a bog-standard PC with capture cards would work, though latency would be a challenge. Or not, depending on your requirements.

>>927271
you're looking for a thing called a multiviewer.
they're about a hundred fiddy bux for a basic unit that takes 2 1080p hdmi in and gives 1 hdmi out with pip and vertical split modes.

120hz frame rate is taking the piss though, there isn't even a standardised format for that.

>>927286
there's no such thing as a format for framerates anyway, not in the traditional sense
It's just the same thing as the usual format, but faster

>>927271
Whats your source? Whats your production workflow? Are you even producing? Whats the target output?

If you're bringing two streams into a computer then open the streams with ffmpeg, use a PIP filtergraph, and pipe the output to a virtual video device to be captured elsewhere