Look at these questions and their answers:
Splitting a row to two rows in LaTeX
split a table cell in two lines and columns
They present similar issues and, similar accepted answers. However, the answer is not exactly the solution to the questions.
I found them because I raised the same question: how to ACTUALLY split cells?
Observe that this is not a duplicate, since I focus on the splitting action instead of merging most of cells. In fact, as a matter of illustration, the merging action is a solution for those working with Excell like softwares, while the splitting action is common for those working with tables in Word like softwares.
Best Answer
You could use nested tabular in order to split cells horizontally and/or vertically: