The following is the answer for A340 and I think it applies to the A330 as well,,,,,,,,
During the last stages of the A340 development campaign, the consequences of the triple failure: PRIM 2 + SEC 2 + GREEN HYDRAULIC, were assessed.
This failure combination leads to the loss of both elevators (and consequently pitch mechanical backup) and the three ailerons (which immediately go to their zero hinge moment position). The resulting pitch up (in cruise conditions) due to the aileron upfloat is very severe and difficult to counter by use of the THS only (failure judged hazardous to catastrophic depending on flight conditions).
As a consequence it was decided to trigger an ECAM warning (F/CTL REDUND LOST) when two of the following failures occur:
PRIM 2 + SEC 2, or PRIM 2 + GREEN HYDRAULIC, or SEC 2 + GREEN HYDRAULIC, associated with a procedure to bring the aircraft to a flight condition where, should the subsequent failure occurs, the pitch up would be easily controllable.
When the aircraft is dispatched with PRIM 2, and SEC 2 operative and obviously the green hydraulic system operative (dispatch with a hydraulic system inoperative is not allowed), two failures are necessary on the same flight to get the F/CTL REDUND LOST warning displayed on ECAM E/WD. A further third failure on the same flight is necessary to be in the situation described above.
Now if the dispatch is made with either PRIM 2 or SEC 2 inoperative, a single failure is enough to get the F/CTL REDUND LOST caution displayed on ECAM E/WD, and a single additional failure on the same failure on the same flight is enough to lose both elevators and three ailerons.
Due to the consequences of such failures it was considered as unacceptable to dispatch with either PRIM 2, or SEC 2 inoperative.
Therefore both computers are NO GO as reflected in current MMEL.