you cannot have a fix assignment of applications to a GPU but to a Desktop. Each Desktop is assigned to a GPU and the application will use the GPU that is assigned to the Desktop on which the application opened the rendering window. Some application will create the window at a fix position, in this case they will always be assigned to the same GPU. Some application don't use fix position but create the window e.g. by calling CreateWindow with CW_USEDEFAULT as parameter for the position. For such applications you can create a shortcut on the Desktop that belongs to the GPU you want to use and launch the app by double clicking the shortcut.
You can also use Hydravision to configure the position/Desktop on which an application gets opened. For applications that use CW_USEDEFAULT this will do the GPU assignment for you.