It is never a good idea to stack filters in this way. It restricts air flow and can cause the air conditioner to ice up, among other things.
Problems related to restricting air flow.
- Reduced operating efficiency
- Increased operating costs
- Evaporator coil icing up
- Reduced air flow to the home
- Longer run times
- Filters could be sucked into the fan causing damage
- Liquid refrigerant flooding back to the compressor
- Possible equipment failure
If the cost of the filter is a real concern, you could possibly install a 2 inch pleated filter into the Aprilaire Spaceguard furnace filter system. As long as the outside dimensions are the same as the Aprilaire filter it should work.
When installing the narrower filter make certain that it is installed against the down stream side (toward the air handler) of the Aprilaire unit. This way, air flow will help to keep the narrower pleated filter in place. This will only work if the filter is mounted in a vertical position. If it is mounted horizontally, this will not work and you will have to figure out a way to hold the filter in place.