2 Air Filters in Air Conditioning Unit
Go for it.
Your only worry should be too much filter resistance, but you have that potential problem now.
You could go for a washable filter as the first layer, then a MERV13 for the second layer.  Or a disposable MERV6 or MERV8 as the first layer and MERV13 for the final.  Expect to wash or replace the outer layer more often.
Your unit may have a sight glass indicating &quot;filter replacement&quot;, if so watch it. If not, you could cobble in a manometer.  Or experiment a bit with cardboard blocking the filter, and get a sense if you can hear the difference in pitch when the airflow is blocked.