I worked for a couple of years on a company where we had a "pair-up all the time" when working and... We were very aware it wasn't possible to keep up like that 8h/day every single day. Most days we were very happy with 4 hours, really productive (and exhausting) days were around the 6 hours mark.

We made sure to take breaks every 45 min-1h30 (depending how well the session was going), those breaks could be from 10 minutes to an hour, it needed to make us feel refreshed, not talk to people if needed, etc.

I actually miss this setup a lot, it was much easier to not feel stuck, much better to have a peer available all the time to bounce ideas, clear assumptions, discover the codebase, etc., but I couldn't ever do it 8 hours/day.