hmm?
SRD:
Entropic Shield
Abjuration
Level: Clr 1, Luck 1
Components: V, S
Casting Time: 1 standard action
Range: Personal
Target: You
Duration: 1 min./level (D)
A magical field appears around you, glowing with a chaotic blast of multicolored hues. This field deflects incoming arrows, rays, and other ranged attacks. Each ranged attack directed at you for which the attacker must make an attack roll has a 20% miss chance (similar to the effects of concealment). Other attacks that simply work at a distance are not affected.
Emphasis mine.
It is not concealment, it is simply describing how the 20% mischance works.
So, one would need to roll both and if either succeeds then you are missed. In essence, they are stacking (but not 'adding').