Question on overwrite and multiagent syncing

Hi. I had a question on the "Overwrite files" flag, which I wasn't sure about, and cant find an exact answer on. Does enabling this flag overwrite each and every file that it finds in the source to the destination, or does it overwrite ONLY changed files based on the checksum?

If it's the former, which options should I choose so that AWS only uploads new and changed files (to accommodate a case where eg. only half the file has been pushed into S3 before the agent went down)?

One other question. Is it possible to have 2 x Agents syncing from 2 different locations into one destination bucket?

Thanks.