A321 flight crew reported while reviewing the MEL's for the fuel quantity indicator; they did not perform the procedure as required per the MEL.

Narrative

Upon arriving at Aircraft X and after completing my pre flight walk around; I was made aware of the aircraft having an an MEL for the LEFT TANK FUEL QUANTITY INOP OR QUESTIONABLE" (MEL 28-XX) with an associated MEL of 28-XY. The captain was reading the MEL procedure aloud and I asked him if we needed to "dip the tanks" and he replied no and I initially agreed that we had a single source failure being the flight deck fuel quantity indicator and that all we would require per the MEL was a fuel slip from looking up the MELs and I did not read the original write ups. We waited for about 30 minutes for this fuel slip and a fueling supervisor came out to confirm what we needed. This caused me to look at the Aircraft log book again and I noticed that the actual original write up for was "REFUEL PANEL QUANTITY INDICATION". The entry stated "REQUIRED MEL 28-XY FOR LEFT WING TANK QUANTITY INDICATION ON FUE PANEL INOP". When I looked up that MEL however I noticed that 28-XY was possibly incorrectly applied and that was for a flight deck fuel quantity inoperative implying that the refuel panel indication was working. This caused a massive amount of confusion between the CA and myself. I believe the correct MEL should have been 28-XZ based on the actual original write up. This difference in MELs calls for different procedures and upon reviewing this event several days later I realized that we failed to properly follow procedure indicated by ball note 2 of MEL 28-XX that states "When the Refuel/Defuel Panel Indication for the affected tank is also inoperative- as evidenced by the concurrent application of MEL 28-XY - Fueler will determine fuel quantity in the affected tank using Manual Magnetic Indicators (MMIs)". To my knowledge we never did this and we thought it was not required. We pushed back from the gate and received 2 ecam's related to the number 2 engine that required the aircraft to return to the gate and go out of service and we wound up not flying this aircraft.Cause: Frustration; confusion; and an erroneous MEL all played a factor in this situation. When one reads MEL 28-XY it implies that the flight deck fuel gauge is inoperative and the original write up (for the other MEL of 28-XX) on further review didn't state whether or not it was the refuel/defuel panel gauge or the flight deck gauge. Based off of this information to my knowledge as a crew we believed that we only had a flight deck gauge inoperative and we could dispatch with just the required fuel slip. I also feel that the specific procedure of measuring using MMI's required for MEL 28-XY should have been a bold item and not in a note in between procedures."