Let us assume that you, for reasons of choice or necessity, are spending time in close quarters with another person. You are then less inclined to visit corona-dangerous locations. In part you are altruistic toward the other person, and in part for selfish reasons you do not wish to lower the common standard of care. If you go to a dangerous location, the other person might decide to do the same, if only out of retaliation or frustration.
In essence, by accepting such a tethered pair relationship, you end up much closer (physically, most of all) to one person and much more distant from the others. You are boosting your locational extremes.
The physically closer you are to the other person, the more easily you can tell if he or she is breaking the basic agreement of minimal risk. That tends to make the tethered pairs relatively stable. Monitoring is face-to-face!
Tethered pairs also limit your mobility, because each of the two parties must agree that the new proposed location is safe enough.
People who live alone, and do not know each other initially, might benefit from accepting a tethered pair relationship. The other person can help them with chores, problems, advice, etc., and furthermore the other person may induce safer behavior. (Choose a carpenter, not a specialist!) Many people will take risks if they are the only loser, but not if someone else might suffer as well.
A tethered triplet is harder to maintain than a tethered pair. For one thing, the larger the group the harder it is to monitor the behavior of the others. Furthermore, having a third person around helps you less than having a second person around (diminishing marginal utility, plus Sartre). Finally, as the group grows large there are so many veto points on what is a safe location ( a larger tethered pair might work better with a clear leader).
Yet over time the larger groups might prove more stable, even if they are riskier. As more things break down, or the risk of boredom and frustration rises, the larger groups may offer some practical advantages and furthermore the entertainments of the larger group might prevent group members from making dangerous trips to “the outside world.”
There is an external benefit to choosing a tethered pair (or triplet, or more), because you pull that person out of potential circulation, thus easing congestion and in turn contagion risk. Public spaces become safer.
As you choose a tethered pair initially, the risk is relatively high. The other member of the pair might already be contagious, and you do not yet have much information about what that person has been up to. As the tethered pair relationship proceeds, however, it seems safer and safer (“well, I’m not sick yet!”), and after two weeks of enforced confinement it might be pretty safe indeed.
Very often married couples will start out as natural tethered pairs. At the margin, should public policy be trying to encourage additional tethered pairs? Or only in the early stages of pandemics, when “formation risk” tends to be relatively low? Do tethered pairs become safer again (but also less beneficial?), as a society approaches herd immunity?
It may be easier for societies with less sexual segregation to create stable tethered pairs, since couple status is more likely to overlap with “best friend” status.
One advantage of good, frequent, and common testing is that it encourage the formation of more tethered pairs.
You can modify this analysis by introducing children (or parents) more explicitly, or by considering the varying ages of group members. You might, for instance, prefer to be a tethered pair with a younger person, but not everyone can achieve that.