I dont agree with Prateek, lets take a quantitative approach to this, lets say we have 1 litre milk and 1 litre choclate, and lets say the cup is 250 ml, then one cup of milk off the milk bottle means we have 750 ml milk in that bottle, then we pur this milk in couclate so choclate has 1250 ml of mixture. then when u take out a cup from this mixture, assuming that this is homogenous mixture formed, the proportion of milk in the cup drawn out is 250/1250*250 = 50 ml while choclate is remaing 200 ml of the cup, now when u add this cup to the milk bottle then amount of milk in that bottle is 750 ml + 50ml while amount of choclate is 200 ml.

Similarly in the choclate bottle 200 ml of choclate is gone so amount of choclate is 800(1000 - 200). and amount of milk let in the bottls is 250 -50 = 200 so both the bottles have equal proportion of thier respective concentrated mixture. so both are equally concentrated even after this mix,

tell me if i am wrong