The one time I remember having a two-day hangover was after a night of serious drinking, not just the usual two or three mixed drinks occasion. A friend of mine won a lot of money in a lottery and he told us to order as many drinks as we wanted. I drank double and triple-sized White Russians, which have a lot of vodka and Kahlua in them. At the end of the night, I was literally tossed out of a car near my house and I had to crawl to the door. I don't know what my blood alcohol level would have been, but I almost lost consciousness.
The following day, I had the worst hangover ever. I couldn't eat, I was extremely tired, and I felt like I was still a little intoxicated. By that night, however, I was feeling a little better. It was the second day that really hurt. My head was pounding, and I couldn't drink enough water to satisfy me. I felt like I had the flu all day. I'd say if you're drinking enough alcohol to trigger a two-day hangover, make sure you plan ahead and stock up on medications and foods you can handle. Don't try to go to work or do any other responsible thing. Just ride out the hangover like you would a bad cold or something.