You need to think of the velocity of the air as it passes through a filter. The higher the velocity, the more frictional effects from the filter and therefore the more work the blower has to do to move the air through the filter. A simple example. Say you had a blower the produced a flow of 1000 cfm. Now assume you have 1 filter that's 24 inches by 24 inches... 2 feet by 2 feet. The area of the filter is 4 sq feet. Therefore to move 1000 cfm of air through that 4 sq feet of filter the air's velocity (speed) has to be 250 feet per minute. 250 feet per minute X 4 sq feet equals 1000 cubic feet per minute. Think of the air as a block 4 feet square on the front and 250 feet long. That block moves through the filter every minute.

Now say you have 2 filters, each 2 feet by 2 feet... a total of 8 sq feet. We still have 1000 cfm of air flow but now the air can travel through the filter at a velocity of 125 feet per minute. We have cut in half the air velocity. And friction is usually related by the square of velocity. So by doubling the filter area we may have cut the wasted energy of filter friction by a factor of 4. And this is only the friction due to air velocity, not the MERV rating.

Simple rule of thumb is sizing filters... 2 cfm of air flow per inch of filter area. You want a very low velocity for the air flowing through the filter. Many installers go the cheap route and only use the filter in the furnace. A filter grille is a return grille with a door that can accept a filter. The more work the blower does circulating air and not fighting frictional effects of the filter, the better your system will be.