I think most of the users have covered a lot of the issues--ADFs are hard to feed due to their poor eyesight, need a varied diet, do best in groups, and are extremely sensitive to changes in water parameters. Often you will see the frog shed its skin in reaction to these changes. As far as space goes, you will see a variety of answers to this question. They have a very low bioload, so from that point of view you could do one frog per gallon easily--but do keep in mind that these guys need space to move around and stay sane, and having a cycled, stable tank is really the best thing for them. Personally, I would not go under 5 gallons for two frogs. The tank should be fully enclosed to prevent escapes.
Another thing I wanted to emphasize is the necessity of keeping track of the temperature of the water. You need it in the range of 70-78 degrees--no higher and no lower. Too hot, and it puts a lot of strain on their bodies, too low and their metabolisms will slow down.