first of all I'd like to thank you for the links you've given to me in the other thread. I will answer soon.
I think there is no difference concerning the common usage of stutter and stammer. I guess if you say "She always stutters and stammers when ..." this is something like "hendiadys" a rhetorical term, which means two words and one meaning. However, I found this:
First off, consider how much easier it is to work with a stammer than a stutter. What's the difference? Some authorities maintain 'stutter' and 'stammer' are two words for the same thing, but I subscribe to the following definitions: a stutter is where the first syllable of the word is repeated over and over like a machine gun, without the second ever being reached. A stammer, in contrast, is where not even the first syllable can be articulated: there's just an ever-widening hole in the sentence. I believe that in this hole, this gap, you can find the silence, the calmness, you need to get the next word out. So, if your speech defect takes the form of a stutter, just some take time out when you get to the hazardous word.
source: Let me speak, by David Mitchell - British Stammering Association
Does that help?