Is it appropriate to drop by another university's professor's lecture to talk about research afterwards?
I would not appreciate that. I have a schedule, which means that after a lecture I may have a meeting, I may have an appointment with someone I am supervising, I may need to pick up my children from day-care, etc. etc. etc.

If you have emailed twice and not received an answer, then that is your answer... That is not very polite, but none of us are perfect.