According to timelines that I have seen, the nurses were not using full PPE until after the patient was definitively diagnosed. So, for two days, they were caring for him while he became progressively more infectious. Hospital guidelines *should* be for full PPE to be used until the disease is diagnosed; at that time, PPE use can be scaled down if the diagnosis warrants doing so.
It's doubtful that the nurses (or anyone else) was infected at that point. First because it was not long after the family contact and they were not infected. Second because it is much more likely that the two nurses were infected when Duncan went acute and they had inadequate protection for that.