ACMS,
The FMC uses the lowest of the 2 ? Then you would never have a way of detecting a fuel leak ! FMC is calculated and totalizer is......totalizer. Well on a Boeing anyway "progress 2 page"
The totalizer has to be the gauge to make decisions on unless its clearly got a technical issue. Once dispatched a brand new 777 with an inop left fuel gauge. Tanks dipped and relied on the FMC only. Not a pleasant experience.