Fuel imbalance is an FCOM limitation, so that is the latest point to initiate GRAVITY FUEL FEEDING.
When the tank with the live pumps reaches 750kg (3000kg FOB), you'll get FUEL L(R) WING TK LO LVL. ECAM asks for FUEL X FEED ON, TK PUMP 1+2 (ON SIDE WITH LO LVL) OFF and will result in GRAVITY FUEL FEEDING from both tanks to both engines.
Obviously not ideal..
[Goldenrivet that section only applies to OEI, but yes you can gravity transfer from one tank to the other]