I've never heard anyone make that argument, but even if they did, so what? If 300 people are in a building, even in different classes, they are, in a sense assembled together, just as they are also assembled together when everybody's in the same room together-but then everybody's not in the same room together if babies, toddlers and their attendents are in the nursery, or if some of the deacons are in the offices counting the contribution, or someone's cleaning the communion trays and cup(s).
The whole thing can degenerates into silliness. Nowhere does scripture say that every single member of the church must be in the same room together for the whole of the assembly. To worry more about who's where in the building than about what's going on where those people are in the building is to really miss the whole point of the assembly IMHO. It is to make the assembly into one more legalistic doctine to argue and divide over. Its elevating the "form of the law" over the" spirit of the law."
And if one cup is mandated by scripture, how many trays/plates/platters are mandated? I guess that would be an inference since trays or plates aren't mentioned. Are trays an expedient? If trays are merely an expedient why aren't cups? This whole argument gets really silly as well when you follow it out to its logical conclusion.
But more importantly, how did we decide we could "abbreviate" the communion feast from a full table meal into a five minutes' passing of trays amidst silent reflection anyway?