I have two tables one is a Client Table and the other is a table containing their partners details. I would like the default values for the partners address details to be the same as the client details. The tables are joined by a primary key ClientID.

This would mean you would have the same information in two separate tables. What happens if the address changes? You would have to go to both tables and change the address in both places. Not a good idea ! You would be better off in having the address in a separate table and have a reference number it in each table for that address. So for example let say you're Client address was 50 , then the default address for the partner would be 50 if the address happened to be different then you would just enter a new address in the address table and link it back to the partner table.