Mass and volume are neither created nor destroyed by being moved around. Therefore if a third solution is produced by combining two existing solutions, then the volume and mass of the third solution are found from the masses and volumes in the initial solutions:
The concentration in the third chamber is based upon the masses and volumes in the first two chambers:
Note that the concentration for chamber #3 must be found from the masses and volumes of chambers #1 and #2, not directly from the concentrations in chambers #1 and #2.
This is analogous to averaging velocities by considering the total distance and the total time.