Depending on what you want in there though, you may be able to achieve that
kind of effect, eg
Format the cell as Chip described, type 'ab' hit ALT+ENTER, type few spaces
and then type 'cd'
Get the spacing right and you will have the appearance of a split cell.
Useless for data that needs to be numeric, but if it functions as a column
and row header then it works fine. Not an ideal choice to me as you can't
refer simply to each value (though it can be done), but it may be what you
are looking for.