Not too much interest here. I guess my question was a little too obscure.
Anyway, I promised to explain how it went down, so it will be searchable if the issue comes up for anybody else. Here's how it played out:
The sale of the first sub-parcel closed in late April. The (buyer's) closing attorney calculated my share of the property tax based on 117/365 days times the 2016 tax bill or ~32% of last year's tax bill. I wasn't able to check the math until I got home from closing, but I had signed a re-proration agreement anyway, so I wasn't concerned about whether it was a little off.
Then, a few days later, the buyer of the second sub-parcel was ready to close. His closing attorney calculated my share of the property tax based on the acreage of the two sub-parcels. In this case, the second sub-parcel was 68% of the total land area, so the attorney calculated that I owed 68% of the 2016 property tax for the first 123 days of 2017.
Add them together and I was bringing 55% of 2016 taxes to closing when I only owned the parent parcel for 4 months of 2017. Since the first closing was done, there was no remedy available there. My attorney recalculated for the second closing, and determined the buyer should pay me for 2017 taxes, since I would probably get the tax bill anyway. The buyer didn't like that, so we agreed that his closing attorney would hold his amount in escrow until tax bills come out and we make final adjustments. Finally closed yesterday.
Lesson learned: If you're in a situation where you are selling subdivided property to different buyers, make sure all property tax prorations are calculated by the same closing attorney using the same basis. Yes, there is a re-proration agreement in place for both closings, so it will be adjusted when the actual property tax bills come out in November, but at least I'm not making an interest-free loan to the buyer's until that time.