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Mono god lists will be the safest way to build the army but I think that going duel gods gives you more flexibility regardless of the drawback. Being able to add faster units to a Nurgle army makes it much harder for the other army to dictate where you can go, similarly putting a Hearld of Khorne on a jugger in a 4 strong jugger unit will add a huge punch that a mono list might lack. Also Khorne cannons of retardedness are soooo good that they will probably turn up in every demon army in a tournament (regardless of the fact that the model is fuck ugly)
Duel god lists are still viable but only to lesser degrees. The only reason I would take them is to help plug holes in your chosen god's lacking area and only if the units are relatively small and cheap meaning you aren't overly fussed if they die off.
The best I can figure is if you took some minor amounts of Khorne with your Nurgle to take those cheap ass cannon's and provide some fire support (by which I mean early on dealing with the large range of new gribbleys GW have been pushing) .
Any larger duel god balances would need to be played by someone who knows exactly what their doing as they balance on a knife edge to an extent. They only exception I can really see is if your game is so large that the let downs in going duel god are absorbable.
Also just like to note that the new Slannesh Herald looks a little like a bored chunky housewife.