You could remove the Yamaha from the picture by connecting the zone 2 output to a small stereo amp, which could then continue to the switch box. Some amps like that may even have an A/B switch like your Yamaha that could let you directly connect the speakers to the A and B outputs and switch them at the amp. eliminating the switch box.

You could remove the switch box by connecting zone 2 to an input on the Yamaha (in which case, you would have two volume controls - the 990's zone 2 control and the Yamaha's control - so you'd want to set one at a convenient position and use the other control) and connecting the two sets of speakers to the Yamaha's A and B outputs.