Technically, the "increase by one stage" language gives us 1 additional benefit over normal expertise die stacking..... the ability to go beyond 1d8.
Normal e die stacking caps at 1d8. However, increasing by 1 step allows us to go to 1d10, whereas just adding an extra expertise die would not. So now the test of that, is there a scenario where we can get 1d10 through elusive + expertise die to AC? So at base we have two parrying weapons + elusive = 1d8. Is there another source of "expertise die to AC" that exists in the rules currently?
There are a few, namely the parrying counter maneuver. Two parrying weapons + parrying counter + elusive is only 1d8 under the "its just an expertise die" interpretation, and 1d10 under the "it increases the expertise die, which ignores normal stacking limits". So there is a small but present niche.
This means the existence of Elusive in no way invalidates our interpretation of how parrying weapons stack.
I don't at all disagree with you, in fact, that's largely where I come out on this as well.
Honestly, regardless of intentions, I do plan to run the features as:
- "Expertise dice", whenever and under whatever circumstances they are gained, will be interpreted as intended to be rolled and the results used to modify whatever value they were indicated as having been gained in relation to, unless being evaluated in the context of a passive interaction in which case the statistical average (rounded up) of the relevant expertise die (were it to be rolled) will be used as a modifier instead of a roll (but this interpretation for passive application does not apply to any Parrying capability, though, as all Parrying features are described as an action taken by the character parrying: an active undertaking to obtain a situational increase to the normally relatively static value of AC, not a passive one).
- "Expertise dice", whenever and under whatever circumstances they are gained, from multiple sources, that modify the same value in the context of applicability, shall increase the size of the largest granted expertise die by one step per other expertise die up to a normal ceiling of 1d8 (but with a hard ceiling of 1d12 able to be reached through the application of any specific abilities that either explicitly state that they bypass the normal 1d8 ceiling for expertise die growth, or that state that they increase the stage of the obtained expertise die rather than actually granting an expertise die, when such an ability logically triggers in a sufficiently late position in the order of operations and the associated resolution of effects).
Regrettably, as applied to the 15th level Duelist dual-wielding parrying weapons (any weapons with either the parrying or the finesse properties, in the hands of that character) in possession of the Parrying Counter combat maneuver, their ability to enhance their Armor Class under my above interpretations is capped at 1d8 of expertise, I believe.
Elusive, the ability that grants the "increase by one stage" feature, specifies that to gain the increased-stage to the expertise die, you must be using your reaction to parry an attack using a Parrying weapon. The only actual mechanism in the game, RAW, that comes close to approaching an ability to
use your reaction to parry an attack (and therefore the only way in which anyone might be able to satisfy the requirement to do so "using a Parrying weapon") is through the use of Parrying Counter, which specifies that you point "your weapon at your foe to challenge them" and thereby gain an expertise die to your AC against any attacks made by that opponent until the start of your next turn.
Technically, under the strictest possible reading, there is no correlation mandated by the Parrying Counter maneuver between the use of the reaction and the use of your weapon (which must be a parrying weapon) to explicitly parry anything; the expertise die to your AC granted against that opponent is not in any way described as you using your weapon to deflect the attack (which is the definition of parrying). That said, the strictest reading of RAW would make Elusive unusable, so we can rely on the fact that the "Parrying" Counter maneuver grants a similar effect as the use of the Parrying property (in terms of the mechanical benefits) to conclude that this is the "use of a reaction to parry an attack" that could be executed while wielding a Parrying weapon, which would trigger Elusive.
Thus, the Elusive die increase happens in conjunction with you using your reaction to (enable you to) parry an attack with your weapon while that weapon is possessed of the Parrying property. However, having used your reaction to (enable you to) parry an attack, you are still wielding at least one weapon with the parrying property. It is not an action, simply an ability to be declared as used in the moment, to elect to add an expertise die to your AC using the Parrying property. This necessarily happens after the enemy has actually initiated their attack, and therefore necessarily after you have already used your reaction to set up your ability to parry with an expertise die (d4, from Parrying Counter) that has been increased by one stage (d6, from Elusive) -- recall, the reaction trigger that Parrying Counter is reacting to is "the end of your turn". At that point, under my interpretation, your election to employ the Parrying property, then, once the expertise die is already at a d6 potential, would bump it to a d8, using the interpretations I outlined above.
But the expertise die from the Parrying property is just that: an expertise die. It has no special caveats that allow it being added to another expertise die that would bypass the 1d8 ceiling. Therefore, the decision, if the Duelist is wielding two weapons both of which possess the Parrying property in their hands (or other manipulating membranes), to parry one attack with both weapons would waste the contribution of the expertise die from the second Parrying weapon, whereas using the second parrying weapon to parry a second attack would grant a d8 expertise die to AC against both parried attacks. (A different d8 to AC, though, as the d8 of expertise against the second attack is from a different source than the d8 of expertise against the first attack.)
(It is possible that my players will prevail upon me to adopt a houserule that rephrases Elusive as starting the relevant phrase with "while" rather than "when", which would provide a measure of discretion to the player as to the point at which the expertise die increase from Elusive is applied, in which case one would assume that a Duelist of at least 15th level wielding two parrying weapons who elected to set up a Parrying Counter and then parry their foe's first attack with both parrying weapons would also elect to apply the Elusive increase at the end to obtain a d10 expertise die.)
But that's just me.
