I assume that by "ventilation fan", you really mean an exhaust vent, not a supply of outdoor air to the indoors?
You're getting condensation because you have humid air going into an un-insulated metal pipe located in an unconditioned attic, where it's cold.
Dripping from a seam or joint in the duct means the duct isn't well sealed either. All joints and seams need to be sealed (with duct mastic, not tape.)
I'm not sure exactly what the duct configuration is. Are both fans feeding the same ducts, that wye or tee in together? If yes, are there at least butterfly backflow preventers on each fan? Or is the ILF250 in series, as a flow-booster for the other fan?
A 150cfm or 250cfm fan into a short section of 4" duct isn't a big problem, but the combined 400 cfm is never going be real. The total duct length (equivalent lengths of all ells & tees) as well as the diameter determines the total amount of back pressure, and the actual vs. rated cfm the fans are delivering. The cfm rating of the fans is rated at 0.10 water inches, but even if they're only hitting half their rated flow, that's not the reason for the condensation. The duct walls are still WAY below the dew point of the humid air that's flowing in them.
Sealed, insulated ducts are the solution. With an insulated duct the duct wall temperatures rise quickly to the dew point of the exhaust air, condensing only a little in the beginning minutes of flow, but then re-evaporating the initial condensate as more air moves through, bringing the duct wall temperature above the dew point of the air. Without the insulation there will be times when the attic is cold enough that the duct walls can never rise above the dew point temperature. Sealing the ducts well is important, since the escaping air will leave it's moisture in the attic or attic-insulation, creating mold and/or ceiling dripping problems.