You should not measure output swing open loop (well, I guess you could, but I would not recommend it). When you apply a large differential input signal directly, you are causing large signal currents to swamp out the cascode transistors. The cascode transistors are going to try to sink/source this extra current, and when they can't, they will go into triode.
In normal usage, you will employ negative feedback to keep the two inputs at the same potential.
See page 6 of the following document for an example:
http://www.mrc.uidaho.edu/mrc/people/harry/ee415/class_handouts/two_stg_char.pdf...