Okay, try this . . .
1) Create 2 "Briefcases" on your Desktop.
2) In the first one, create and save your Excel document.
3) Right-click the Excel document icon and select "copy".
4) Double click the second Briefcase on your Desktop, right-click again, and paste your Excel document.
Now, you go into the second "pasted" Excel document and make the changes you need. When you want both Excel documents (in both of the different briefcases) to record the same information, then type into the first Excel document you created and save it. The go into the second Briefcase, right-click on the Excel document icon, and selct "update". What will that do?? Well, it duplicates the changes from the first one onto the second Excel document, leaving whatever you had there before as it was.
Hope this works for you!! Let me know if you have any questions. I know it is kind of comlicated, so you might not end up wanting to do it this way.
A Fellow Questioner, Anne