OK, multiple sources of concealment don't stack, but what if you have some other source of a miss chance (the non-invisibility portion of Blink, for instance)?
You roll each (non-concealment) miss chance separately. You then roll one (the highest) concealment miss chance.
However as DM, if there are multiple concealment miss chances, you can give a bonus to the concealment miss chance (say, a 20% for Blur and a 20% for medium fog, you could make it a 30% concealment miss chance).