EDIT: the above also certainly makes sence. I think the FCOM description of the message that Microburst2002 reminded us of, i.e. "the FCOM DES FUEL ECAM warnings says that the caution is triggered by discrepancy between FMC and FCMC, not between FMC1 and 2..." is a rather basic description of the process behind the warning, which, for the sake of simplicity, only addresses points 1 and 2 in Romasik's post and doesn't address the more technical third point:
1- the value of ZFW sent by the FMS differs between side 1 and side 2 of more than 100 lbs,
or
2- the value of ZFWCG sent by the FMS differs between side 1 and side 2 of more than 0.025 %,
or
3- during the initialization/re-initialization process, the status of ZFW/ZFWCG sent by the FMS differs between side 1 and side 2 during more than 2 seconds.