Uhm Seeing as i dont have dawnguard, i dont think i understand what this glitch is. Could someone explain it better?
If you are a vampire or vampire lord, the thieves guild hood will not render correctly nor is it marked as using up a head piece slot. So in this case you are able to wear the thieves guild hood along with another piece of head gear.
I used to wear the ancient shrouded hood but I switched my look up a little more to be a cross between assassin + necromancer. I wear the forsworn headpiece (female) for the horns (current avatar picture).