"Manifestar" is a word which calls for a whole range of different translations in different contexts. I think the factors here are (1) it is contrasted with "comunicar", and (2) it's very desirable, indeed pretty well essential, to use the same word in both places here.

I'd say that "express" could be a viable option. "Manifestar" is to reveal or make known. It implies making a statement, and specifically articulating your feelings or views. "Express" covers this fairly well, I think. You could say "what we wanted to express" in the first case and "we hope to express ourselves by keeping silent" in the second.

I also considered "declare", but I don't think it works so well here. For the second case, where manifestar is used intransitively, you could say "declare ourselves", but that usually means propose marriage, and I think we should try to avoid having to resort to a verbal phrase like "make a declaration" or "declare our feelings/opinions". "Express ourselves" would cover it, I think.