I want to to my RaspBerry Pi 4 with an old small CPU fan. However it's a 12V fan. Is it possible to use 2 of the 5V pins on the board to total to 10V? Sorry if this is a dumb question. I haven't worked with electronics in a while.
This is not possible as the two 5V pins are connected together and both referenced to the same ground potential.
Either you should look for a 5V fan or get a boost dc-dc converter to get to the required voltage.