The potential Nylander/Marner have is too high to trade him for someone that isn't a #1D. And the issue is that if you did make a move like that you're giving up 5+ years of control as there arent many, if any, #1s under the age of 25. The alternative is moving them for someone who looks like they are going to be a #1 (ala Werenski, Parayko, etc), but then you also risk them not panning out as that bonafide #1 while Nylander/Marner end up becoming what you believe they can be (or vice versa, or both not living up, or neither living up).
There are a handful of D I would move one of Nylander/Marner for, and they aren't available so there isnt a deal to be made (Karlsson, Subban, Hedman, etc).