ratio of the fill level of the first reservoir to that of the second reservoir deviates from the ratio of the capacity of the first reservoir to that of the second reservoir this allows the dispensed amounts to be controlled by the controller in such a manner that the amounts intended to be dispensed are maintained in a subsequent dispensing operation if the ratio of the fill level of the first reservoir to that of the second reservoir does not deviate from the ratio of the capacity of the first reservoir to that of the second reservoir however if it is established that the ratio of the fill level of the first reservoir to that of the second reservoir deviates from the ratio of the capacity of the first reservoir to that of the second reservoir then the amounts intended to be dispensed can be changed by the controller for the subsequent dispensing operation it being alternatively possible to change only one amount to be dispensed or both amounts to be dispensed thus the amounts to be dispensed can be adjusted in a manner that allows the first and second reservoirs to empty at substantially the same time even if emptying has occurred unevenly because of tolerances in the dispensing of the treating agents as a result of which an amount different from a predetermined target amount has been dispensed accordingly the controller exerts a compensating effect to achieve simultaneous emptying and thereby allow the user to refill both treating agents in one operation in one embodiment each reservoir is provided with only one sensor and each sensor monitors a discrete fill level with a predetermined residual volume in order to have sufficient capacity to compensate for the dispensing error the residual volume.