In some cases this may even be the stated purpose: partners (one or both) may openly proclaim that they want to see other people to relieve emotional or sexual frustration, and/or to reassure themselves that their partners are truly the ones they want to be with.
The explicitly temporary nature of the separation implies the hope of eventual reconciliation and renewed intimacy within the relationship, but the experience of intimacy with someone else during the separation may only make that reconciliation harder to achieve, because that hope may seem less sincere.
(Ironically, this may imply that couples may find it easier to reconcile after a "permanent" separation—one with no set ending date—than after a temporary one, especially if one or both partners saw other people in the meantime, simply because with the permanent separation there is no expectation of reconciliation and less feelings of betrayal to overcome.) Let's learn something from Ross: a "break" is not a "break-up," and if you are in a temporary separation, remember the ultimate goal is to get back together with your partner.
I don't usually see that much concern about being honest to the person outside the two-some.
It would be nice to see concern about the ethics about how the 'other' is treated.
As with everything within a relationship, it's up to the partners themselves to decide what they're comfortable with during the separation, especially regarding how much and what kind of intimacy in dating is allowed.