There's no simple solution here. There are multiple approaches you can take to link the two starter solutions.
This project is not a trivial undertaking. Be prepared to go slow and look things over a lot before making any changes.
The primary key field in ContactManagement is K_ID_Contact. The primary key in Customers is Customer ID. When finished you will have replaced every reference to Customer ID with a reference to K_ID_Contact or you will have done the opposite.
You have two similar tables: customers, in the Invoices file and ContactManagement in the ContactManagement file. When you are done with the process of merging these two files into single solution, you should have one such table that links into both the tables in Invoices and also the tables in Contact Management.
To start the process you can either double click the contact management table occurrence shown in your screen shot above and then use Add FileMaker data source to point this table occurrence at Customers in Invoices, or you open Invoices and re-point the 4 customers table occurrences, Customers, Similars, Invoice Billing Customer and Invoice Shipping Customer to the ContactManagement table in the Contact Management table.
Both approaches can be made to work. Before doing so, you'll need to compare the list of field definitions to see which takes the smallest number of changes to get a table that works equally well in both layouts. You'll need to add field definitions from the table you choose to discard to the table you choose to keep. Once you make repoint the table occurrences, you'll then need to update the relationships linking that table occurrence to the others to use the fields specific to the new table. Then you'll need to open each layout in that file that refers to it and update any field references there. Finally, you'll need to review the scripts for any needed updating. If you have Filemaker Advanced, this becomes easier as you can generate Database Design Reports that you can search for the key words "missing" and "unknown" to track down any loose ends.
However you work with this, keep original copies of both files handy for comparision purposes.