Glendald, If your exisitng laminate is uneven, it will not be a suitable subfloor for your new laminate.  I'm not sure exactly what your case it, but you said you want to even the floor, and the laminate should not be used for that purpose.  Dips or high spots in the exisiting laminate will transfer to the new laminate over time.  Another thing to consider is that due to the height of putting more laminate on top of the old laminate, you may have to cut your doors to fit.