Its not really a cough, its a respiratory infection, the cough is just a side affect of it it seems.
You really need to work on changing lots of things with your husbandry. Im sorry but there is lots wrong.
if anything please read through a couple of these
http://www.beardeddragon.org/articles/ and read through the enclosure section on the forums so you can help out your beardies.
Crushed walnut is very bad for beardies. It would be better to house them on newspaper, paper towels for the mean time (both of your beardies) Slate tile is amazing. It costs probably $7 to get enough to fill a tank and it will never have to be replaced unless you break it or something. You just wipe the tile clean after a poo and your good to go. Unlike how walnut shells/sand is basicly just like a literbox your beardie is living in.
Get your beardies basking lights and it would help if you picked up an acurite digital thermometer from walmart to check that the basking temps are in check.
Do you dust your feeders with calcium and gutload them before they are fed?
There is really so much info to cover and its really difficult to cover it all. Please read through the different sections on this forum it will help you out tons!
Good luck (sorry if im sounding harsh but it doesn't sound like the beardies are living in optimal living conditions. Im not blaming you either since you probably heard wrong from a petstore but head my advice, read and learn from this forum
)
-Brandon