Vauny is dead right!
The RCCB will trip if you have an overall earth leakage current of 30 milliamps. Each of the two circuits may have, say 20 mA (milliamps) leakage, and combined they will cause a trip to occur. Although theroetically "safe" at this level, any significant earth leakage current should be investigated and rectified. Both of the circuits you mention involve water, but I would suspect the sewage pump as the main contributor as it is less likely to be kept clean and dry. Please do have it checked by a qualified tester for earth leakage.