Riddle #1318

Question: Swallowing one is fine.
Swallowing two together is fine.
But Swallow them separately and all shall fall.

What are they?

Question: What do you call a sad bird?

Question: What has a bed that you can't sleep in?

Question: How are doughnuts and golf alike?

Question: What can be broken but never forgotten?

Question: People see through me, but none pass through without killing me. I can be anywhere, and can be seen double or single. What am I?