Are there only 5 levels on the split screen coop mode?

Yes, but if you have xbox Live you can also download 2 more.

There are actually only 4, but 5 if you count the "Training" excercise as a mission. Two additional downloadable missions, the complete list of them is as follows:

Training
Panama
Seoul
Chemical Bunker
Train Station

Nuclear Plant
UN Headquarters

To get the d/l ones, go to the Co-op screen then Xbox Live and finally downloads. It will list them there and NOT on the Live/downloads from the main screen.