I was thinking of trying [There's Nothing to Fear!"] alongside ["They're On Fire!"] on my Imbagon but I'm not sure if the damage reduction stacks or they have to be used alternately.
I think you are confusing [skill]"Save Yourselves"[/skill] with [skill]"There's Nothing to Fear!"[/skill]
As far as the OP question, I'm not 100% sure but would guess wiki has an answer. If I was to guess, I would assume diminishing returns. For example: Player would take 100 damage from *whatever*. TNtF is up and reduces 35% of that down to 65 damage. They're on Fire would then reduce that 65 by *30%*(or whatever its at) bringing the damage down to 45 damage received. I could be wrong though so take that with a grain of salt.
For the record I am using Save Yourselves on my Paragon. I just want to find a way to reduce even more damage during the recharge of "There's Nothing To Fear!" (I have Leadership at 14 giving it 11 seconds duration), and to see if it stacks for additional damage reduction, to compliment the way I play alongside Heroes and Henchmen.
The build setup is something like this:
[build prof=Paragon/Warrior Lead=12+1+1 Spea=12+1][Aggressive Refrain][Focused Anger][Anthem of Flame][Spear of Fury][Save Yourselves][There's Nothing To Fear][They're On Fire][Signet of Return][/build]
My current Sunspear rank is Commander (6) and my Luxon rank is Warden (8).
Also, given the way you describe the damage reduction would work, is it safe to assume that it would also reduce the damage in order of activation?
I think if you had 30% reduction from TNTF, and the 30% reduction from ToF at the same time you would have a total of 51% damage reduction. 30% is taken off the 100% base damage, and then the 2nd 30% is taken off the 70% remaining damage. So the effect diminishes as you have more different effects.
Its not diminishing returns. Lets assume a player has 600 health.
At 0% damage reduction, they take 600 damage to kill.
At 30% damage reduction, they take 857 damage to kill.
At 51% (total) damage reduction, they take 1224 damage to kill.
Therefore, 30% damage reduction lets you absorb about 42% more damage over 0% damage reduction, and 51% damage reduction lets you absorb about 42% more damage over 30% damage reduction. The amount of protection offered by damage reduction goes up linearly. If it actually stacked such that 30% + 30% = 60% it would be exponential returns (and be game-breakingly imbalanced).
TntF + ToF works great for anyone who is tired of being the SY bitch of the group.