is there a simple way to connect the 5V pins of the Arduino and the raspberry pi in series to get 10V ? Thanks
Yes, you could just theoretically connect +5V pin from Arduino to Raspberry 0V pin, and so you could obtain 10V when probing +5V pin of Raspberry to 0V pin of Arduino.
But DON'T DO IT.
Besides of getting two points that can be measured as 10v potential with your multimeter, this setting will be of no utility at all.
You can't connect Arduino to Rpi (first because Rpi uses 3.3V logic, second because what Arduino thinks as "digital 1", Rpi would see as digital 0.
This is a really nonsense and there are a dozen practical reasons for not doing this in a real world situation. There are other ways of doing whatever you want to do.