1. I haven't seen you and Helen for a long time.
2. We can't stop typhoons and earthquakes from happening.
Are the above sentences acceptable?
Yes, they're OK. The first would be more common if you usually see "you" and Helen together, eg. if they are a couple.
Originally Posted by sitifan
For 2. either 'and' or 'or' is possible; no one's going to nitpick that.