Transferring data from one column to another automatically?

Column A and B are going to continuously be updated. I need to create a formula that 1) searches columns C and D for the contents of Column A and B for each specific row e.g. I need it to search for A2 and B2 as a combination (not search the columns individually) and 2) copy any data entered in Col A and B to Col C and D if the data isn't found in C and D. If the data is found nothing needs to be done.


Thanks

Attach some sample data. Also attached the expected result.