That is the best way I know of. With the encoders and pads on the Axiom Pro there are four groups or virtual sets of controllers, but the sliders and numeric pad only has one set. If your 2nd Gen has groups of sliders, that would be the best... With the virtual groups of pads, for example, I just press the soft key under the display to switch between groups - making 32 controllers available in the same patch.
Now, if you can live without sliders (using them for volume controllers?) and use encoders for that purpose, and also live with less than half the available encoders, you
could assign those 18 encoders (or more) to the 18 tracks in Sonar. However MIDI only allows 16 channels, so that's another consideration in your 18 SONAR tracks... are some on the same channel to get 18, or do you have different MIDI channels assigned to each of your 2 patches on the Axiom?