Definitely would like to know, myself. Ristretto means “restricted” in Italian, right? And some say it’s half the time of extraction so 15sec or sometimes even 10sec in some cultures. And a ristretto single espresso is shorter than a normal single (around 3/4 of an ounce), so with that logic, my guess would be that a double ristretto seems like it would be something around 18g dose / 20± seconds / 45± g out. Idk. What I do know is that even pros may not agree on what an espresso is or should be.
To further complicate things, when you split a double shot through a double spouted portafilter into two separate cups, should you call those now single shots? But that will taste different from a single shot traditionally made with half the amount of coffee, right?