My question is very related to this https://discussions.apple.com/thread/3204532.
I just want to do the same but in merged cells, as in the picture below. Is it possible?
I see no fill handle here.
Fill handles do not operate with merged cells. You can use a formula to achieve the same effect.
Insert the following in these cells:
A1 (the merged cells A1 and A2)
A3 (the merged cells A3 and A4)
Copy and paste A3 for the rest of the column.
The following animation shows the entire procedure: