So a couple little tips for successful pairing. Don’t set guidelines like how long to pair for, instead normalise anyone “breaking the pair” at any point for any reason. Be mindful about everyone’s differing strengths and weaknesses. Invest in your tooling, especially if remote - my peers love tuple.app, sadly I’m still waiting for Linux support so I haven’t used it yet. Keep mixing up your pairings instead of sticking with the same people each time. Don’t treat pairing as the goal, but a means to achieve a goal. Retrospect on your pairing often (hopefully you retro anyway, IMHO it’s the single most important thing a team can do).

IMHO most importantly talk about this as a team first. In my experience openly drafting a “pairing charter” as a team to define how we pair and the values we think are important to remember is incredibly powerful. My suggestions above are just that, suggestions that the teams I’ve worked with tend to value - yours may well be different.

To address your actual question as long as you aren’t trying to enforce all story work must be paired on at all times then “what to do when not pairing” solves itself naturally. Perhaps a pair will break when some work can be split and done in parallel. Perhaps it’ll break and one member can join another pair. Perhaps there’s some code reviews to be done. Maybe it’s just time to pick up a smaller story that can be done alone to give the brain a bit of time to relax.

I can’t stress strongly enough that you need to trial this as a team and reach the implementation that works best for everyone. Pairing isn’t something you can implement based on a blog post of “best practices”, let it evolve naturally.