Only one of the two missions - "Fool's Gold" (Yevhen) or "Finding Nathaniel" (Nathaniel) - will fire in a given playthrough. If the game's
Origins save import registers Nathaniel as having survived
Awakening, you will get "Finding Nathaniel". Otherwise, you will get "Fool's Gold".
The import, however, was bugged, such that Nathaniel would register as "dead" if:
1. The Warden-Commander chose to save Amaranthine instead of Vigil's Keep.
2. Nathaniel was not in the Warden-Commander's party for the battle in Amaranthine and the Dragonbone Wastes.
Even if the Warden-Commander upgraded the Keep fully and got the epilogue slide talking about Nathaniel's survival, the import would still register him as dead. That meant that lots of people who kept Nathaniel alive in
Awakening still got the quest in
Dragon Age II for when he was dead.
I'm not sure what the reasoning was b/c I always did Nathaniel's but I think the story reason was that Nathaniel got the Deep Roads info from Yehven somehow maybe? And thus didn't need to talk to Bartrand, and thus Delilah never overheard anything, so she never sought out Hawke.
That is a very shaky recollection at best, however and may be wildly inaccurate.
Nah, that wouldn't make any sense.
People who've used mods to enable both missions have noticed that they are badly bugged when this happens: enemies and characters don't spawn properly and such things. Yet, obviously, the stories are not contradictory: it doesn't make any sense for Yevhen and his sons to not ask for help just because a Warden they don't know from a country they're not from is alive. Plus, if you enable "Finding Nathaniel" after playing through "Fool's Gold" and saving Merin, you can actually see Merin in Nathaniel's party - he's become a Grey Warden.
Given that amount of interactivity, I think that the original concept was clearly
not for the quests to lock each other out. But during development, I'd guess that the Deep Roads environment was bugged: they didn't have time and resources to build separate dungeons, so they put all of the Deep Roads stuff in the same place...but the game had trouble differentiating the Deep Roads in Act II from the Deep Roads in Act III, so they had to go with a quick fix and have the quests lock each other out.