I have used the word ‘adjoining’ to refer to items on the same horizontal plane – adjoining bedrooms in a hotel; adjoining fields; adjoining houses in a terrace. The neighbour each other and share a boundary.
‘adjoin verb (adjoined, adjoining) to be next to and joined to something. adjoining adj. ETYMOLOGY: 14c: from French ajoindre, from Latin ad to + jungere to join.’
Chambers Dictionary
The third and fourth floors in a building would not have been described by me as adjoining but according to Chambers Dictionary, they could be. I just never thought of adjoining in a vertical plane- until the Nagle brothers in Kilavullen last week.
I get that the coffins may be next to each other vertically in a single plot but I have not yet reconciled how they they share a common boundary, how they are joined, except in death.