How do I identify changes or updates?

Here's my issue:

I am trying to identify what changes someone has made to an earlier version of a worksheet to update changes.

Basically, two people are typing notes and changing values on different copies of the same spreadsheet. Now we need to combine the sheets without erasing either person's work.

I am trying to avoid slugging through a manual comparison of 20,000+ rows of data and notes. Any help would be appreciated.

I can use this at the begining of a project when we share a spreadsheet, but how do I now combine two different spreadsheets without deleteing one or the other person's work? I was hoping I could look at one spreadsheet and see what the differences were in the other, but I can't find a way that is possible.