heads up : whilst two camera and viewports is not that hard , be aware that keyboards in general have limitations , perhaps you are planning to use joystick input ?
I cant confirm this with facts, but I think this problem exists only on some keyboards or for certain keys. If a keyboard stops working after 3 buttons, try using other 3 buttons, this has worked for me in the past
... I just love NFS:HP...
PS.: Too bad I deleted all of my old projects. I made an awesome split-screen shooter in a dungeon setting once