To do a "James's Dad" is to purchase two different items from the menu and share. This allows you to try two different things without paying over the odds...
The term came from visiting a restaurant and overhearing a family on an adjacent table. The young boy was called James. He wanted the same dessert as his brother but his Dad suggested that they all have something different and share to ensure more variety!
This we thought was a good idea, we now do a "James's Dad" everywhere we eat! Try it!
A: "I can't make up my mind whether to have the fudge cake or the apple pie?"
B: "Neither can I"
A: "Wel how about I have the fudge cake and you can have the apple pie, then we can do a James's Dad..."