The best situation for the leos is to house them separately for 2-3 months, to ensure that both are healthy before housing them together. You wouldn't want to infect a healthy one by housing it with a sick one.
You'll want two humid hides (or at least one that's large enough for both, but I think two might be better). I've used both opaque and translucent humid hides, and my leo didn't mind either.
As for substrate in the hide, damp paper towels are perfect for young leos. You may want to avoid things like moss to minimize the chance that they swallow something. Once mine was older, I used a bed of damp moss covered with damp paper towels. This combination kept the hide humid for longer, as the moss held onto moisture and released it into the paper towels slowly.
Also, placing the humid hide near the warm half of the tank will allow for more humidity.