This depends on the size, stock and your PWC's.
While there's no such thing as too much filtration, there is such a thing as too many filters. I see filtration as converting ammo to nitrates and polishing up as much as possible. If you have a large tank with really messy fish ... yes you want 2 or 3 filters. However, I feel at some point though, adding more filters will not improve bio filtration by adding more bacteria. They multiply in response to Bio-load. Larger Bio-loads = more ammonia = more bacteria.
Too many filters and you'll end up creating a ton of current.