OK OK OK, let's back up!
Both Denshield and DensGuard have a protective surface on the face of the board. THAT is the first line of defense and THAT is the surface that must be respected. When it comes to seams and fastener penetrations they can (and will) render the surface "not waterproof" anymore, unless those breaches are repaired. Caulk, silicone, RedGard can be used for the repairs and covering the seams after they have been filled with thinset.
Now, the second line of defense with those products is the interior of the board. It is made of components that are not effected by moisture. I have tested Denshield and found that to be 100% true of that product. Even if moisture should get through the surface-cover the moisture won't hurt or deteriorate the board's interior. BUT, you still don't want moisture getting through so you repair the breaches and seams.
NOW, the surface is already waterproof so you wouldn't want a moisture barrier behind the boards also to create a "moisture sandwich" trapping moisture between the two water barriers. If the surface is however truly waterproof then condensation can still be an issue in there.
HOWEVER, to apply additional waterproofing (RedGard) to the already waterproof surface of the board will not have any effect on the interior of the board and will not create the feared "moisture sandwich". (Bytheway - I think that is a stupid sounding term, I hate it. Had nothing to do with the creation of the term "moisture sandwich.")
Do I have us all totally confused now?