The tunnel is just wood, not treated but I make sure to keep it dry.
The enclosed is kind of heavy due to all the substrate in it so my dad and I had to reinforce the table due to it sagging a bit but other than that there have been no problems with it (Except that its quite messy now...
Different species can make each other sick if they are kept together, so it should be avoided at all costs.
I would not recommend keeping multiple tortoises together even if they are the same species although if you choose to keep those together be prepared to separate them at any time.
Does bubbles come out of his nose at any time or is it just when he is soaking or drinking? Is his food moist so that he could get water in his nose or mouth? Bubbles coming out of their nostrils when drinking or ingesting a lot of water is normal.
Fluid buildup in the lungs caused by sickness...
You should also read this: http://www.tortoiseforum.org/index.php?threads/Box-Turtle-Care-Sheet.93937/
It contains everything from housing to diet.
If you have any questions after that just ask around on the forum.
Though I do not own a box turtle I can still give you some advice.
Box turtle like humidity, therefore it is recommended that your enclosure is not open topped.
What kind of substrate are you using?
How warm is the enclosure at the basking spot, warm side, cool side and during the night...
The enclosure seems rather small for the two of them, they should also have more hides.
I do not know if the water under the enclosure works but it's almost impossible to maintain humidity in an open topped enclosure.
I think I've found a problem with it...
He can see his reflection in the walls and I think he is trying to attack his reflection :/
The problem is that the reflection is on the inside of the enclosure so the material on the outside doesn't help.
Coconut coir works well I've heard, just don't get the zoo med tortoise house. It's expensive and not suited for most tortoises.
It's quite easy to make an enclosure for a baby, as they don't require as much space as older ones.
The thing about tortoises is that they require a lot of space...