I just finished an oatmeal stout, and used a pale ale for the tan.
The key is to have one beer finish at a higher FG than the other. My stout finished at 1.020, and the pale ale was 1.010, so the stout was the bottom and the pale was the top. I used the "bass turtle", like a spoon, to pour the lighter beer on top. Here's a picture:
The pale ale looks darker in the picture- it was pretty light colored.