I might be wrong but I think the only way you can sort of make it "stack" is by being the caster of Warding Bond. The target of the WB gets resistance against all damage types and transfers the damage they take to the caster of the bond, if the caster then also has resistance against a specific damage type they should be doubly protected, but only against the damage they soak from their ward. (Chain-casting WB doesn't work afaik.)
That is most certainly not how is this supposed to work.
That is it working RAW.
"While the target is within 60 feet of you, it gains a +1 bonus to AC and saving throws, and it has resistance to all damage. Also, each time it takes damage, you take the same amount of damage."Damage is taken only after mitigating factors have been applied. So things like Resistance, damage reduction (From things like Heavy Armour, Heavy Armour Mastery, Arcane Ward etc) reduce the damage of an attack and you only receive the end result.
This is then transferred via the Warding Bond to the caster of the Bond. Where again, mitigating factors apply before taking the damage.
The skill does not make both parties take the same attack, it does not mention that the transferred damage bypasses mitigation, it does not change the damage in any way.
It simply takes whatever damage that is taken by the recipient of the Bond and creates a new source of damage of the same value as was taken and directs it to the caster of the Bond.
Hence the caster of the Bond can "Double Dip" into Resistance/Damage Reduction effects by having their bonded target have them as well as using them themselves.
In reality, the actual quirk of this spell is that the damage to the caster is counted as if it came from the Bonded target. So if the Bonded target has effects like the Caustic Band, Callous Glow Ring, Elemental Adept etc. These effects are applied to the damage that gets sent to the caster. Which is more of a Larian coding quirk than a RAI scenario.
