Yes that's right.
Sample_A = Drug1 (12 concentrations) x Drug2 (12 concentrations).
Repeat for Sample_B, Sample_C, Sample_D.
I average them all into a single matrix based on the % of cells that die (20% for example). Then transform that into a synergy score (how well both drugs perform compared to what you would expect based on each individual drug alone).
So if I get a single cell of the matrix that has a synergy score of 50 (for example), but all other cells of that matrix have scores on the order of ~1-5, that should mean the concentration of Drug1 and Drug2 that give the score of 50 is by far the best.
How could I put a p-value on that cell (containing a score value of 50) ? Saying compared to all other values of the matrix, it is statistically different.