There’s 4 variables.
Concentration X, Concentration Y, Crystal Z and Crystal W
Adding concentration X to Z and W gives me two equations:
X+Z -> XZ
X+W -> XW
Adding concentration Y to Z and W gives me two more equations:
Y+Z -> YZ
Y+W -> YW.
I can then use the systems of equations to solve for all 4 variables, once I know the concentration of acid required to dissolve YW and YW. To keep it easier, I make sure I have the exact same amount of both Z and W, and I keep the concentration in the standard solution the same. The only thing that will change between the two is the volume of the acid.
How did you standardize the acid solutions (i.e. how do you know what their molarities are)?