Have just realised, there has been no guidance on this. For infection control purposes, presumably separated parents should both isolate if one of them has symptoms or is awaiting test results or tests positive.
But this could create bubbles of several / many households, in the case of blended families being involved. Especially if any single parents in the "chain" have bubbled for social support with another family as well.
It could also lead to conflict if one parent's new partner works in a high risk job / is more cautious or whatever - the ex may not be happy about themselves having to isolate, and perhaps lose pay, "because of him!".