Microburst
You have a point. Most posts take it for granted that you must continue your FL and the flight to destination. Maintaining cross feed till impossible situation develops and then try to find a solution does not fit in any decision making tools. Initially when the failure occurs do the ECAM but after that a situational decision needs to be taken from your options. As Tubby elaborated the scenario if you were above FL300 then you can descend to FL300 and stop cross feed. That is the point you were asking WHEN TANK FUEL RQRD. Failed one side pumps and low fuel on the good side due to cross feed is definitely not an option. A diversion needs to be considered. As you rightly pointed out there is no cross feed procedure on two engines. If your company can accept a flame out on good pump side due to starvation and gravity feed on the live engine side then you can try all these methods.