] I had been under the impression that he did know that she knew -- but I'm not sure where that impression came from. So I reread chapter 35. This time, I could not tell whether Edward knew that Elinor knew about his engagement or not; I thought his awkward manner could fit either case.

I think that it's more a matter of what is not said rather than what is said that provides the clues. I don't think Edward has seen either of these ladies from the time Lucy told Elinor of the engagement until the moment the three are together, so I don't know why he would think Elinor knows.

In Ch. 34, Lucy laments to Elinor that she has not yet seen Edward since he came to town, but then--although she sees Elinor nearly every day if not every day--there is no exultation that they have met at last. Considering Lucy's past behaviour, I would imagine that she'd tell Elinor immediately.

Later, in Ch. 35 after the dinner party with Mrs. Ferrars, Lucy is exulting "...there is one good thing--we (Lucy and Edward) shall be able to meet, and pretty often..." To me this is another clue that she hasn't seen him up until the moment he walks in on Elinor and Lucy together.

Edward has also tried to see Elinor twice--or at least he left his card there. That gives me the impression that maybe he meant to tell her in person, if he could.

Of course, Lucy might have written to Edward that Elinor knows of their engagement, but it was meant to be kept a secret and perhaps he would not have been pleased with her for telling it, especially not to someone who has other connections with his sister.

When Edward walks in on them, it also appears from Lucy's behaviour that she hasn't told him.

This paragraph, in particular, gives the distinct impression that Lucy has not told Edward about letting Elinor in on the secret:

It was a very awkward moment; and the countenance of each shewed that it was so. They all looked exceedingly foolish; and Edward seemed to have as great an inclination to walk out of the room again, as to advance farther into it. The very circumstance, in its unpleasantest form, which they would each have been most anxious to avoid, had fallen on them -- They were not only all three together, but were together without the relief of any other person. The ladies recovered themselves first. It was not Lucy's business to put herself forward, and the appearance of secrecy must still be kept up. She could therefore only look her tenderness, and after slightly addressing him, said no more.

It would also be just like Lucy to want to watch Edward tell Elinor himself, don't you think?