Add two surfaces to vtmkscripts.vmtkSurfaceViewer

I’d like to compare the pre and post smoothed surface from the marching cubes script. Here is part of my code. I’m building my own pypes script, so I am trying to replicate what is found in this tutorial at the end: Prepare a surface for mesh generation | vmtk - the Vascular Modelling Toolkit

surface = vmtkscripts.vmtkMarchingCubes()
surface.Image = voi_selector.Image # image is loaded from CT data and the voi_selector crops the region of interest using vmtkscripts.vmtkImageVOISelector()
surface.Level = l
surface.Connectivity = 1

surface_viewer = vmtkscripts.vmtkSurfaceViewer()

smooth = vmtkscripts.vmtkSurfaceSmoothing()
smooth.NumberOfIterations = 10
smooth.PassBand = 0.1
smooth.Surface = surface.Surface
surface_viewer.surface = smooth.Surface

What I am trying to do is just add both surfaces to the same viewer. Additionally, I am selecting various locations in my image (by looping through different bounds using the vmtkscripts.vmtkImageVOISelector() and I would also like to see them all at the same viewer. Currently, I save the intermediate steps, and open them in a Paraview, but wondering if I can do it all using the viewer.

Thanks in advance,