Quad buffered stereo can only be set before or with the viz.go command. You could try the following to go from quad buffered stereo to mono mode but I don't think it's possible to toggle back:
Code:
import viz
import vizact
viz.go(viz.QUAD_BUFFER)
viz.addChild('piazza.osgb')
# Press spacebar to render in mono mode
vizact.onkeydown(' ', viz.MainWindow.stereo,viz.STEREO_LEFT)