I have two different spread sheets that share data points that I would like to link so that I only need to enter data in one to update the other. Is this possible? And if it is possible do both spread sheets need to be open at the same time?
yes, that's generally very easy possible. Let's say in Workbook-1 you enter the data and in Workbook-2 you want to have the link to the respective cell in Workbook-1:
The easiest way is to have both workbooks open, enter the = sign in A1 of Workbook-2 and then just click in the corresponding cell in Workbook-1. It will create the link for you.
Now let's say, later on you change the value in Workbook-1, save and close it. When you now open Workbook-2, it will notify you that there is a link to an external source:
Depending on your choice, it will update the value or not.
Just one word of warning:
External links might are risky. If you rename, move or delete the source file (= Workbook-1), the link will break and you will get errors when you update. Especially if you share your files with other people that do not have access to the Workbook-1, they also will get errors.