Momentary button for voltage boost circuit design
Putting two different power supplies in series will most likely lead to the failure of one of them, due to power mismatching.
But for a simple non-permanent demo it should work (for a little while).
You could also do something similar with batteries.
If you are as new to electronics as you say you are, I'd probably not go down the path of the charging up a capacitor for a motor of this size. It'll get complicated fairly quickly.
While not as fun as a motor, you could do something with an automotive incandescent light bulb.  Power it with 8V then make it go brighter with 12V. That would be easier to achieve with a capacitor charge/discharge system.