I would like to run a computation in one NoteBook that will take 2 hours.
Could I assign this Notebook a specific Kernel so I can run computation in other Notebooks ?
You can select which kernel is used by your notebook from the menu item Evaluation -> Notebook's Kernel. By default you will probably only have one kernel called Local available. If your Mathematica license allows for it (typically licenses allow for two simultaneous kernels on a machine), you can add new kernels by selecting the Evaluation -> Kernel Configuration Options... menu item. Select "New" to add a new Kernel, give the kernel any name you want and accept the default options. This new kernel will now be selectable from the Evaluation -> Notebooks Kernel menu.