furnace filters

That would actually make it clog up twice as fast. To make a filter last longer you need more surface area. Making the filter twice as thick does not add any surface area.

Well, lets see... You have to change it when it gets dirty. Would putting a second filter behind the first stop the first from getting dirty. No? Then why would doubling up add life?

Does the manufacurer recommend doubling up, or is the blower designed for a single layer? What do you think doubling the air resistance will do to your system?

If you could double up by doubling the area (rather than thickness) you would be on to something.

Oh yes. Doubling the thickness does increase the filter surface area. It also increases the air resistance to recirculating air and that may reduce hot air reaching registers further away from the furnace. You probably won't lose the total amount of heat output from the furnace. I think having the filter clog faster might be a desirable result as one can throw away a clogged filter and thereby permanently remove the dust from the circulating air. In fact I think I will do this, cut the hammock type filter to increase the regular filter's effectiveness(?)