It is taken for granted that the sensor is a good one! ie swapped the 1st and 2nd over and still have the same problem?
If no cat is fitted between them the second sensor may be trying to tell you the cat is not working, good of it to notice.
You can buy lambda sensor eliminators if the reason that the sensor is giving a true and realistic message that you do not want to see.
The other technical alternative is Technical Topics (
www.techtopics.co.uk) Bridgwater, James will know what its telling you and likely how to fix it, once its hooked up to his kit that is.
JohnC