How is this? [If there were] two women, this one having two pairs [of obligatory bird-offerings] and this one having two pairs [of such of-ferings], [and a bird] flew from this one to that one, it renders one [bird-offering] unfit when it leaves. [If] it returned [to the original group], it invalidates one [bird-offering] when it returns.  [If] it flew [again from the first group into the second] and [then] it returned [again from the second group into the first], [then again] it flew [from the first group into the second] and [then] it returned [again from the second group into the first], nothing [more] is lost, because even if they are [all] commingled, no less than two [pairs will be fit to offer].