The first sentence is correct. The second sentence is incorrect. You could say “I ate 2 strawberries” or “I ate 2 crates of strawberries” but you cannot say “I ate them up twice.”

You can say "I ate them all up in two sittings."

I guess 'twice' isn't a good measurement even in Chinese. 更确切的说吃了两天，或者两个人吃，或者说一次吃不完可能更合适？That would be I ate it for two days, OR It takes two to finish it. OR I can't finish it in one go

In the way I understand what you said, I think you had eaten the three pounds just in two rounds

If you eat something twice, something very bad has happened.