If I have, f.Ex., two items with 20% chance to affect 2 extra targets, grimtools adds that up to 40% chance to affect 4 extra targets. Same for DoTs applied to skills.
Do those skills actually stack the way it is listed?
How would this work with, say, resistance reduction?
Everything is additive, so if there’s 10% RR in one weapon, you’re dual wielding, you have 20% RR. Chances and duration doesn’t stack.
Example:
20% of X over/for 3 seconds
15% of Y over/for 5 seconds
= 15% of X+Y over/for 3 seconds and 20% of X over/for 3 seconds. The effect that lasts longer will be extended, so after 15% X+Y expires, Y will continue for 2 more seconds.
Except if both RR come from the same weapon 
(dual wield Fang of Chton for example).
Ok, so when it combines 2x 100 dmg over 2 seconds to 400 dmg over 4 seconds thats just a calculation error.
yes it is a bug
Oh, one last thing:
2 weapons count as a single source, but 2 rings?
Or a weapon + a piece of jewelry? I assume that’s different sources, and thus does not stack?
(Example:
2x Mythical Mindwarp + 1 Mythical Eternal Band)
It’s about skill modifiers, as far as I understood, so this is why I didn’t mention that at all. That’s a skill in item, not “bonus to skill”, as OP called it.
I have a question though. So take for example kiliran’s mace - the one with 20% WD to FS.
If I DW them…do I get an additional 40% WD from each weapon? Or just 20% from each?
20 from each, for a total of 40.
Ok, just want to confirm.
So if FS base damage is 130%, 2x mace = 170%.
So each weapon will therefore deal 170% WD, yes?
No, each weapon will hit for 169%. 
Yes, 170 each.
Thanks user_name. Has anyone ever tried a pure physical FS with beronath reforged + mace?
Ah, one more question:
If I have an item granting a bonus, say, RR, to a summon skill, which I assume will actually grant that RR to the summoned minion, will an active ability that temporarity grants RR to my minions, like Bysmiels Command, stack with that?
Can you give a specific example with a GT link? It’s hard to tell without looking at the wording.
https://www.grimtools.com/calc/4VxOR4M2
Granted, I have no idea if Bysmiels Command actually applies to minions.
Bymiel’s command applies to everyone, but no it will not stack.
It counts as a different, independent source of X RR.
So basically, anything not attacked by hellhounds will have 20 RR applied, but anything attack by dogs will have 36 RR.
The latter should happen more often than not seeing that hounds have that aura to them.
Hold up, I thought flat RR did not stack regardless of the source?
Is this an exception?
And is it because one of the RR gets added to base while the other becomes sort of a buff?
They stack if you stack them on the same skill via skill modifiers.
So does damage reduction. I.e. in the past, it was possible to DW 2x black flame dagger, and give FoI 36% damage reduction to enemies.
I do remember something of that discussion about the dual-weapon-to-skill stacking of damage reduction, though. Do you have the link?
This might need testing to confirm if flat RR behaves the same, but likely does as to-skill usually stacks.
He’s got 2 veilpierces each giving Hellhounds 18 RR.
So dogs apply 36RR, but not the 20RR from bysmiel’s command.
All other pets buffed with Bysmiel’s command ought to apply 20 RR to any target they attack.
However, this 20 RR immediately gets overridden should a hellhound attack this aforementioned target either directly, or with AoE attacks.