If the drivers are in series, then there will be one wire going from one to the other, kind of like this: +-()-()--
If they're in parallel, there should be two wires connecting them. But why do you care? The total impedance is all that matters, if you want to put the two 12"s on the Pathfinder.
When you say it won't be as loud, what do you mean? Not as loud as the Solos through the two 12"s, or not as loud as the Pathfinder through the 8"?
If you try this, make sure that the Solos amp is not connected to the speakers when you're powering them with the Pathfinder.