Forgive me for asking but I am not sure if my furnace uses one or two filters?
Which leads me to the question of placement, if just one filter is needed.
My furnace is a coleman 7900 series gas unit (heating only) installed in a mobile home.
One the inside of the blower housing is a number of 7945-590 not sure if that is a specific model number or not.

Although I am not a professional furnace person, I would think ONE would do,but I'll look for the model type you have and make sure.

KBC