You need to be a lot more specific when it comes to Discus. What do you mean when you say young discus? What is their current size, does couple mean 2 or more.
A 40G is fine for 2-3 inch Discus as a grow out. And I wouldn't suggest getting only a pair, as Discus when they are small prefer to be crowded as they feeler safer and less stressed in a group.
You will need to do daily water changes, have a lot of filtration and feed them multiple times a day. Keep in mind that tiny Discus have very weak immune systems and hence need very clean water. So it will be quite a challenge.
Minimum tank size suggested for fully grown Discus is a 60 gal, though the bigger the better, obviously.