So according to that, PtL and EI do not stack.

Yes they in fact do. Because the trigger for both is after performing an action, which is the same trigger for getting a stress.

Since you have the ability to decide what order you resolve effects with the same trigger/timing you can chose to take the free action from PtL or EI before you get the stress from either one. We've been through this on the rules form, and even had Frank's ruling on it via email.

So it looks like this...

Trigger Dauntless

Perform free action from Dauntless

*After Action* trigger

Trigger PtL

*After Action* trigger

Trigger Experimental Interface.

Gain stress from Dauntless

Gain stress from PtL

Gain stress from Experimental Interface.

Hmm.. that seems highly counterintuitive: I can see how multiple abilities trigger from the same event, but it would seem logical to first finish one ability before going on to the next, thus getting a stress token at the end of either one of those actions and then not being able to do any more.

But here I stand corrected. Thanks for clarifying that. It's always good to learn something

Hmm.. that seems highly counterintuitive:

That was my reaction at first as well. I figured you had to finish the whole of Push the Limit, before you could go on to the next thing. But once Buhallin explained how he saw it, I saw the logic in it. Then we sent it off to Frank at FFG and he confirmed that it did work this way.

Yup its described in the most recent FAQ. If an ability has the same trigger, it can trigger from the same event and interrupt the first event from resolving completely, allowing a chain of actions and a stack of stress.