Is it okay to use two 20x20 air filters instead of one 36x20 for my home AC?
In order for your air filters to work correctly, they must be the exact size made for the filter box otherwise yes, dirt, dust, etc. will get sucked right through the gaps. Even if you were to seal the gaps with duct tape, it would only last a little while before it came off. Your AC system will last much longer and be more efficient which will save you more money than the little savings you get from the previous occupants cheap fix.