John Gill Commentary Exodus 26:24

"And they shall be double beneath, and in like manner they shall be entire unto the top thereof unto one ring: thus shall it be for them both; they shall be for the two corners." — Exodus 26:24 (ASV)

And they shall be coupled together beneath
At the bottom of the boards or planks:

and they shall be coupled together above the head of it to one
ring
at the top of the boards there was a ring, to which they were coupled and fastened, and so at the bottom of them, though not expressed: some understand this of all the planks, both at the two sides and at the west end; but it seems rather to respect only the corner planks, since it follows:

thus shall it be for them both, they shall be for the two corners
which were coupled and joined alike at each corner as if they were twins, as the word used signifies; or the meaning is, that these were double boards, and so like twins, which were so closely put together that they seemed but one board; which was done that the corners might be thicker and stronger, and so for the greater firmness of the building.