My name is Joan and I have been a Vet tech for 30+ years with special interst in Reptiles and Reptile rescue.
Can you tell me where they are housed inside or outside?
If inside do you have a UVB light? how old,tube or coil, strength and age of bulb?
Do you feed any supplements?
is the problem with the shell or is it the slin also?
The UVB is getting old at 5 months. We replace them every 6 months. What I suspect you are seeing is the shell getting ready to shed. The translucent or shiny parts of the shell would be the the shedding starting. all reptiles shed and a tortoise is no different, they shed their scutes, which are the large areas on the shell. You can read about the shedding here: http://www.peteducation.com/article.cfm?c=17+1797&aid=2700
The other reason the dull shell could be a bit of dehydration. I would give each tortoise a good soak at least a copule of times a week to be sure they stay well hydrated. I like to use Dioralyte electrolyte replacer 50/50 in the water. Always keep a low pan of water for the to soak themsleves. The pet supplies do carry a shell conditioner that may be good for both the tortoises. I would also leave a cuttole bone available for a calcium supplement which is also good for their beaks. Joan