Does anyone know if Horizon Air and Iceland Air have a baggage agreement neither airline can/ will tell me?

I booked my flight with Iceland Air but when I received my itinerary it said that my first flight would actually be with Horizon Air. When I contacted them they said I would have to wait until I got to the airport on the day of my flight to find out if or what the charge would be and that I might even have to claim my luggage in Seattle before boarding my connecting flight and it's possible that I will be charged baggage fees by both airlines. HELP!!!