Thank you for asking about bearded dragon and housing them with a leopard gecko. The simple answer is no they should not be housed together ever. The main reason being is the bearded dragon will eat the leopard gecko once he is a bit larger than it. even if you could avoid that for a time, there are still many other reasons that this is not something you want to do.
Bearded dragons have a very high metabolism and are hungry throughout the day, they will attack and eat another lizard of any species, including their own that they can fit into their mouth. Even though leopards are nocturnal and beardies are sun lovers, there will be times they are both up and active and that will make the leo dinner sooner or later.
Each species requires different temperature to thrive at, different light/UVB needs, substrate and diets, that can cause problems for the other species. Additionally bearded dragons have parasites that are unique to them and considered a normal part of their digestive fauna, but that parasite can be deadly to a gecko as that lizards gut composition can be deadly to a beardie. An example is coccidia, which is considered normal in low levels in a bearded dragon, but can kill a leo.
Please let me know if you have any further questions about this