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Merge or unmerge table cells in Pages on iPad
Merging table cells combines adjacent cells into a single cell. Unmerging cells that were previously merged retains all the data in the new top-left cell.

Here are some restrictions:
You can’t merge nonadjacent cells or cells from different areas of the table—for example, cells from the body and the header.
You can’t merge columns or rows.
You can’t split a cell. If a cell has never been merged with another cell, it can’t be unmerged.