The modification i suggest below assumes you're inserting 'Customer' and 'Fake Time' manually in the model. If you're loading 'Customer' and 'Fake Time' from Data Hub or via Anaplan Connect, then you can just run a process (with 3 actions) that inserts 'Customer', 'Fake Time' and 'Customer/Fake Time'. Essentially you're adding new members in all 3 lists, so that the same 'Customer A' is created in both 'Customer' and 'Customer/Fake Time'.
Do see if the following work:
1. I've added 'Count' line item in all 3 modules.
2. A new module that compares the number of 'Customer/Fake Time' against 'Customer' + 'Fake Time' is created.
3. In the dashboard, you can check if 'Load Customer/Fake Time?' is true or not, if it is then you can click on 'Add Customer/Fake Time' process.
3. The process is made up of 2 actions, one of which is the creation of 'Customer/Fake Time' list from 'Customer' list. The 2nd action is the creation of 'Customer/Fake Time' list from 'Fake Time' list.