It's perfectly normal for them to be stressed, for a couple of reasons. Firstly, they see all that open space as a chance for predators to get them. Adding floating plants to hide them from birds, and lots of decorations to swim through, hide behind or sleep in will help them feel safe from other fish who might eat them (I know there are no other fish, but they don't. :p) Secondly, they are used to being stuck in an itty bitty little space, so their muscles aren't yet strong enough to cope with lots of space+filter flow. They feel weak, and that again means that they are susceptible to predators.
They will get used to all the space, but definitely do pack it full of decor. Live plants are great, if you can get them. :) As for the cories, that's a no-no. If it were an undivided tank I'd say go for it, but cories need the full length of a ten gallon, as they are active little zoomers. :)
If you need to prevent the two boys from seeing each other, you can do a couple of things. You can make a central compartment for the filter between the two boys, which also acts as a kind of air-lock if one of them jumps the barricade, or you can just line the divider with plants to break up their line of sight. :)