SSRS Merge Data Across 2 Group Values
It is a bit kludgey, but here's an idea sketch... instead of merging the cells, could you fake it by placing an A in the left cell aligned to the right and a D in the right cell aligned to the left?  You could then use expressions to control value, alignment, color, and borders of the two separate cells.

Let me know if you need more details.  I think this would give you the visual you want.  Good luck!