In theory, it would have the same effectiveness as a single filter as a given parcel of air will only pass through a single filter with both designs. To increase the effectiveness of the filtering, you would need to place the filters in series, ie. have two (or more) filters on the back of the fan. However, this would increase the resistance of the filter block and the fan would operate at a lower flow rate. So there’s a trade-off between effective air filtering and effective air flow.
In theory, it would have the same effectiveness as a single filter as a given parcel of air will only pass through a single filter with both designs. To increase the effectiveness of the filtering, you would need to place the filters in series, ie. have two (or more) filters on the back of the fan. However, this would increase the resistance of the filter block and the fan would operate at a lower flow rate. So there’s a trade-off between effective air filtering and effective air flow.