Well, it could be debated, I mean it doesn't say in the PhP that the effects cannot stack. My DM ruled that the effects could be stacked when a new but brief player joined our group playing an elvish bard, but she happened to have a piccilo if I can recall, while my bard had a lute. Now, different types of musical instruments have different types of sounds, playing together producing harmony, and thus producing a stacked benefit. So in short, it's up to the DM's ruling, but the whole stacked benefit of multiple bards seems feasible.