Hello and welcome to LinuxQuestions,
Have you looked at the 'Displays' settings? If both monitors show the same then most likely you have 'Mirror displays' checked in there. If so, remove the 'check' and click Apply. In the same preferences window you can change the resolution and position of your displays by clicking, holding down and moving the boxes that represent your displays.
Looking forward to your participation in the forums. Have fun with Linux.