I'm not sure there is a safe way to do what you are proposing with those tanks. The pressure on the seam between the 2 may be too much for it to take, let alone what may happen if the tanks are ever bumped.
You can try breeding the 2 (1 male and 1 female) in a 20 But won't be able to have any other fish in there. Do keep an eye on them as the male may or may not allow the female to live peacefully in a tank that size. Providing lots of drift wood and caves (more than just 2) should help to curve any aggressive territorial behavior from the male.
Other than that all you need is is good food, good water quality, and a male and a female who are of age.