HKS says Anna said at the funeral that she wanted to go with Daniel.
Ok Anna was worried about bugs getting into the coffins. 2 double plots were purchased - Maybe those 2 double plots were for just Anna and Daniel. If she didn't like bugs then most likely she like the idea of 2 caskets being stacked on top of each other into one plot and that may be why she bought 2 plots do avoid double stacking of the coffins. Just because the cemetery allows double stacking of coffins does not mean it has to be done that way.
I wonder if they require concrete inground vaults so that grave does not sink in.
Personally I would never want a double stacked plot with my husband's coffin under my coffin or on top my coffin. I would rather rest side by side with him. (this includes any of my children). Put me 6 feet under but not 10+ feet under.