Hello ~
One mandarin in a 20 gallon tank is a disaster, let alone two. That's not fair to the fish to do that...people think that just because they see bugs and lots of life in their tank that it can sustain a mandarin, and you know what? A small percentage of the mandarins switch to eating dead food but the vast majority of them eventually starve and die, and they take a good long while to starve to death, too. If your tank is only a few months old, just be patient and let the pods build up a sustainable population before putting a fish in their that eats hundreds of pods a day ~ buy some good bug kits (such as from Inland Aquatics or Indo-Pacific Sea Farms), give your tank a bare minimum of 6 months to mature (better yet, give it a year), and don't add other fish that will compete for the mandarin's food and decimate the pod population, like wrasses and scooter blennies and gobies, etc. And please, please don't think about putting one in a 20 gallon tank.
I usually don't get involved in threads like this, but you really need to hear from people who have seen this same tragedy play out over and over and over and over. Different person, different forum, same story.
~ Sherri