If I were doing this, I would book each segment under separate PNR's to insure separate credits for each segment. However, as the BWI-ISP segment is the last flight of the day, you run a risk of having to upgrade to Y fare the next morning if your PBI-BWI flight is delayed and you miss BWI-ISP.
If they're under a single PNR, I suspect you may only get the (lowest possible) credit for a one-way flight PBI-ISP. Just my gut instinct, worth only 2 cents - or less.