IMO there is not much benefit in having two small wet/dry's as opposed to one big one. If you used two, you could use one for mechanical/chemical and the other for mechanical/biological but there is no reason you can't do mechanical/biological/chemical filtering in one big one. Redundancy may be one point for using two, so if one pump breaks, you have another one to keep water moving until you replace the broken one but the more complicated your system, the more likely is it that something will fail. So I would say stick with one big wet/dry and an HOB or canister.
