You can combine both layers' extents and increase a bit the resulting bounding box to make sure all geometries and labels are visible.
In the following code, I i) take first and second layers from the ToC, ii) combine their extents, and iii) scale the resulting bounding box by 10%:
extent = QgsRectangle()
extent.setMinimal()
layers = [ iface.mapCanvas().layers()[0], iface.mapCanvas().layers()[1] ]
for layer in layers:
extent.combineExtentWith( layer.extent() )
extent.scale( 1.1 ) # Increase a bit the extent to make sure all geometries lie inside
iface.mapCanvas().setExtent( extent )
iface.mapCanvas().refresh()
Best Answer
You'd get the app's Map Canvas and set its extent.
You might want to read this tutorial.