I am not sure I have fully understood the issue,please let me know if the below helps.
First off, There is no way to use a formula to populate the Code field of a list. That being said:
A - If the second list you have mentioned (the numbered list) has items with are already a concatenation of A & B as items (and thus the problem is that FINDITEM does not work with the name in numbered lists) I suggest you to create an action to import all item's concatenated name as their code too:
Just go to your list, hit import, select the same list as a source and map "display name" (which i suppose that is the property that contains the concatenated name) to Code. Use the list's names as identifier.
That will provide you the concatenated names as codes. Then you can publish the action on a dashboard so you can trigger it in order to keep the list's codes up to date (in case that A or B changes, in this way you can keep the code aligned with the new name by triggering the action)
Unfortunately you cannot schedule actions within Anaplan. In order to automate the action, you would need to create a batch file that executes that particular action through Anaplan Connect. The way to run the BAT file automatically is to use a scheduler, such Microsoft Task Scheduler.