When you split a table, Word inserts an empty Normal paragraph between the
two parts. Assuming you have made no other changes, you can just delete this
paragraph to rejoin the parts of the table. Where you will run into problems
is if you have marked any rows in the second table as heading rows, or if
either of the tables has (intentionally or inadvertently) become wrapped
(formatted with Around text wrapping) or in some cases if column width has
been changed, columns or cells merged, etc.
