This would be more visually appealing than one big wheel. The challenge with two weels was how to somehow couple their rotation together. At first I wanted to just cut a grove in the curcumference of the wheels and use an O-ring style belt to drive them both, but I couldn't figure out where to find that kind of belt. The next idea was to just cut gear teeth into the wheels. This is a lot of work, but more visually appealing than a belt.
Visual appeal is very important, as the marble machine, after all, has no real purpose.
I then made a second wheel exactly the same diameter. I didn't have another piece of plywood of the right thickness around, so I used thinner plywood, and then glued some pieces of wood onto it to bring it to the right thickness. The pieces I glued I cut from a piece of firewodd. Then I cut out the space between every other set of pencil marks with a bandsaw. This has to be done with great precision.