Does Legacy's Super-human Durability and Fortitude interact?
The order is up to you.
Under the original rules.
If you play under the original rules, resolution happens in order of whichever card was played first.
Under Conflict Resolution in the standard rules, it states:

Over the course of a game, there are situations in which multiple effects would occur simultaneously or have competing effects.  In all situations, card effects that happen simultaneously occur in the order in which the cards enter play.

The &quot;oldest takes effect first&quot; rule is consistently applied to resolve other simultaneous conflicts, even when they are on the same card.  For example, in the clarifications to La Capitan's &quot;Split Across Time&quot; ability in the advanced rules:

La Capitan’s “Split Across Time” side has an ability which reads “whenever la capitan would be dealt damage, move a card from under this card to the appropriate trash. If you do, prevent that damage”. Her advanced rules on this side say “the first time La Capitan would be dealt damage each round, redirect that damage to the non-villain target with the highest hp”. Which effect should trigger first?
Because of the order in which the abilities are written on the card, La Capitan would prevent damage by removing cards from beneath her until she could no longer do that, then use the damage redirection ability.

Thus, if Fortitude was played first, damage to Legacy would be reduced by 1 before Super-Human Durability takes effect.
But, if you don't like that, then you pick the order.
The whole &quot;conflict resolution&quot; section was designed to keep the game moving smoothly.  In some certain cases - and Superhuman Durability versus Fortitude is explicitly called out - resolving in order can make the game more complex or less fun.  Since the whole intent of the rules is to ensure players are having fun, players are allowed to resolve ambiguous situations as they deem fit.  So you can choose the order each effect is applied, as long as the cards are not &quot;start of turn&quot; or &quot;end of turn&quot; effects.
(Christopher is one of the developers of Sentinels, so that's about as official as you can get.)