My first job was at a small OB unit very similar to what you are describing. When our L&D unit was seperate from our postpartum unit (and actually, the unit was for postpartum patients and non-infectious med surg patients), we had a primary nurse that stayed in L&D no matter what. If there were no patients, she would stock, clean, work on chart packets, etc. If one patient came in and she needed help, or a second patient came in, she would call the secondary nurse. The secondary nurse, if not needed, was on the med/surg unit usually acting as med nurse, they never had their own patient assignments in case they got pulled. We would also have a tertiary nurse who would actually take a patient team, usually the postpartum patients, and they would have to find relief for her if she was needed in L&D.
When we changed to LDRP, we kept 2 nurses minimum, and there were times when we might not have anything to do. Rare, though. We took care of our own instruments up to the point of sterilization, so you could usually find some instruments to scrub and then run through the instrument washer. Or, as before, stocking, cleaning, putting together chart packs, working on continuing education. What finally forced me to leave my much-loved job was when they would try to force us to work alone. Their reasoning was that we had a nurse who lived 5 minutes away who agreed to be on call for us, but that wasn't good enough for me. If I had a prolapsed cord, how would I call for my help?