Short answer: No.
The information from one of the cells will be used to fill the merged cell.
I won't go into all of the rules, but with 2 adjacent cells being merged, the
value of the cell in the top or left cell will be retained.
What you can do is copy the cells to another location, merge the cells you
want to merge, then copy from the copies you made to add to the data that
remains in the merged cell to put it all together again.