I own DUNWITCH HORROR and DARK PHARAOH but never mixed them up to avoid diluting the theme; is this a legitimate concern or am I nitpicking? I kind of felt that it'd be like watching two movies at once...

You know, it's your choice :)  If you're comfortable playing this way, no problem!  As far as I noticed, though, you wouldn't get dramatic dilution with DH and CotDP mixed together.  Hm, try it once, you can always spearate them afterwards as needed. :)

Fecktor said:

There's less chance of a gate in Dunwich and if there are less gates, there are less monsters and less monters means less chance of a monster going into a vortex and calling the Dunwich Horror.  I'd call that significant dilution.  Also because less cards of the CotDP will come up there will be less chance of Exhibit items.  While this might be less significant, it certainly reduces the thematic possiblities of CotDP.

I no longer mix the Mythos cards of expansions but randomly determine which expansion and which herald to use.  However the monster mix is from all the expansions.  Even this might cause a problem. Today we played DH and AH with the Whateley prophecy and Yog Sothoth.  Still the Horror didn't come close.  Four gates appeared in Dunwich as well as several monster surges calling monsters there.  At one point there was six monsters on Wizard Hill.  But since most of them were yellow, blue or green they didn't go anywhere near the vortices.