A 20 gallon tank is not big enough to keep two adult discus in,the minimum tank you could get away with is 40 gallon IMO.Discus get quite large (around 8" diameter) and need to be fed large ammounts of food to remain healthy,as said by SMB they need perfect water quality at all times and you will find it very difficult to keep the nitrates down in a 20 gallon tank,you would need to be doing water changes at least once a day and maybe twice.
As suggested try to start out with some smaller easier to care for cichlids,there are many dwarf cichlid species which have stunning colours and are more suitable for a 20 gallon tank,take a look at some of the Appistogramma species if your interested.