X2 creates a
main out for each pair of hardware outputs enabled in preferences. You create a m
aster bus (or use the default Normal template) which points to one of the main out pairs. At least I would imagine most do not send tracks or buses other than the master bus and heaphone mixes directly to the main outputs. If all the audio resolves to a master bus changing the output of the master bus to a different main output pair would be a snap.
IOW a master bus is just a special case SONAR bus (using the Set as Default Bus option) in the bus section at the bottom of the track view and the middle section in the console view. The main outs are representation of the physical hardware outputs in the far right-hand section of the console view. Route all audio through the SONAR master bus and point the master bus to a main out.