Absolutely [member=57948]ChuckS[/member] As I see it, none of those "alternate" hook-ups are viable.
Series, would just reduce the suction even more, to what end? A small amount is going to get through even then, so you would never eliminate the filter or collection bin.
Parallel, really? You have already lost performance by having two volumes of air and all of the related connections/restrictions, then you add another inlet point? to a single source? Yet, they claim increased CFM? Still just one hose going into the vac, they "T" together, which is the worst way anyway. All this will ever do is decrease static pressure. (suction)
Multiple vacs to one hose? Nonsense. Even if it would work, the complexity and requirement of two separate circuits to run it would be prohibitive, ignoring the cost factor. For the cost of 2 vacs and 2 DDs, you could have what is really required for larger tools, higher volume of air movement in the form of an actual dust collector. Even the cheap, entry level models will work for a single tool.
They are louder and generally don't appreciate quick on-off cycles of something like a miter saw, but if you turn it on and let it get going, they will handle you table saw or planer (thicknesser)
Extractors work on speed of air movement, collectors work with the volume of air movement, and move more chips/dust along with it.