So rumours are stirring and I stress rumours!
DHMG is possibly only an advisor!
given that many people are say that cullen is an advisor (not confirmed by bioware)
this may make sense. supposedly Cullen is a military advisor
meaning that DHMG is probably logistics/money advisor, which would make sense in that it creates conflict between them on more then one level
power vs logic
templar vs mage
generic blonde hair vs Moustache
the only problem with this I see is that in my head cannon Dorian is so witty and intellectual that its not really a fair fight.
Cullen: this is a major strategic point we should garrison troops here and re enforce the area, so it cant be taken!
Dorian: Inquisitor in an ideal world mr cullen is correct but I'm afraid the Inquisition is in, Dire need of finances there are far more pressing issues we need to address, we can find another location for the men to polish their armour but we need funds NOW!
Cullen: Your FUNDS! are useless if we cant hold this location, the enemy will attack if we are to hold this ground we need steel!
Dorian: and how do you intend to supply the men with steel without gold? Harsh words and rocks? or maybe you can try swaying them with your winning personality?
Cullen:.............and what good is your coin when an army at the door? how do you intend to defeat an army with gold and sliver?
Dorian: Iron Bull, how much money would it take to say, have you rip off the......Former, Knight captains arm and beat him with it?
Iron Bull: More then you have Mr dorian.
Dorian: But there is a price?
Iron Bull: every thing has a price.
Cullen: 
Dorian: and how much would it take to divert your band of mercenaries to harass the near by town thus preventing the enemy from deploying a large number of troops and leaving the town unprotected?
Iron Bull: substantially less.
Dorian: You see mr cullen? problem solved.
Cullen: wait.... Iron bull you would really charge this mage more to cause me physical harm then risk the lives of your own men?
Iron bull: of course.......If i harm you i owe the Inquisitor a new military advisor.. AHAAHAHAHAHHAHAHAHAAHAHAHAHAAHAHA
Dorian: 
Inquisitor: 
Cullen: 
Iron bull: HAHAHAAHAAAHAHAAHAHAHAHA *walks out of the room* AHAAHAHAHAHAHAHAHAHAAAHAHAHAHAHAAH...