The Max 40% text majes me think the answer is no, but if that only refers to the skill it is listed on them maybe it could stack. Still not sure.
The skills stack multiplicatively iirc
So Carnage Amatsu takes 40% DR at max, and then Spurn would take another 40% of that for a total of 64% DR, or 0.36 of the original damage taken
It’s similar to stacking dodge effects
It can stack like all other DR skills, but DR effects are less effective the more DR you stack on top of the other. PM1 has mentioned stacking spurn for a total 64%DR in his video Yen’fay builds on YouTube
Yup, it stacks multiplicatively. So you get .6 ×.6 for a total of .36, or a total of 36% of the damage you have have taken
Units like B-Ike and Harmonic Myrrh can do the same thing (using Spurn in B-Ike’s case and Dragon Wall in Myrrh’s).
However, if you’re fighting a foe that has forced follow-ups, NFU is better than a DR B slot. Because 2 hits of 36% damage is still more than one hit of 60% damage
It’s not optimal, however you can indeed stack the DR from his Prf+B slot.
However, if you plan to give Yen’fay the +10 treatment, if using him on Summoners Duels, having B slot DR + Adroit Captain could actually prove to be his best set on that mode, since other B skills like Lulls, NCD or Trace could prove to be less valuable than extra +24% DR.
For every other possible way and game mode of using Yen’fay, NFU is his best B skill.
It stacks, and does so multiplicaly.
The first 40%from his weapon gives him 1.66 times as much effektiv HP.
Secound 40% from B-slot results in 2.77 as much effektiv HP.
Making it scale stronger the more you stack it.
That said like others have said NFU is a very solid skill to use.
And depending on support and the current meta you use him in, it is most likely better.
But if the meta rellies less on guaranted follow ups and negation of them, taking an extra 12% of attackers dmg (0.36*2 - 0.6) might be worth it for the extra 1.11 effektiv HP you get in all other fights.
In general I would recomend NFU and one or even two flayns as support, that focus on drive atk and spd, over distant/close guard