Pic

One should be enough. Two may be too restrictive for air flow.

It could be that the cabinet is set up to accept a one inch or two inch filter. You would have to remove the side tracks to insert a two inch size. That is probably was easy at the time of installation. Never use two filters. A single good quality Merv 8 filter is sufficient.

Is this cabinet sitting on basement cement floor? How does the air flow through the filter?

Agree, a single 2" filter would be best. I'd shoot for MERV 13 if your system can handle it, otherwise as high a MERV rating as it will handle. Lot's of info online about MERV ratings, PMx, and why higher MERV ratings are important for your health.

There seem to be different opinions among HVAC contractors concerning what kind of filter is best to use in a forced air system. I have two homes, in two different areas, both mild weather areas where heating and cooling are not heavily used or stressed. I have had two different but both terrific and continuously trained, familiar with current thinking HVAC contractors to work with (each put in new equipment for an existing home).

Start with the premise that for replaceable standard filters, their job is mostly to keep dust and other air floating debris out of furnace (air handler) and coil air passages. With that in mind, Contractor #1 believes that even though they're a bit restrictive with airflow, the typical hanging element accordion shaped air filters are best. I don't know what the MERV value is but it must be in the teens.

Contractor #2 thinks that because most existing systems have ducts that are too small to begin with, any restriction of airflow puts an unneeded added stress on an already stressed fan and also reduces system efficiency. He recommends the thinnest, fiberglass woven filters with single digit MERV ratings - like 7 or 8 as I recall. I bought a case and I'm not there right now so I forget the specifics.

Take your pick, flip a coin. In each location, I use what the local service provider recommends. It's their job to maintain the equipment, I do it their way, even though the two are different.