these terms are definitely debatable and espresso brewing has evolved quite a bit since the old italian days when terms like ‘doppio’ (double shot) and ‘ristretto’ (narrow / reduced) were coined, as they relate to espresso. so do the terms still apply to what we’re doing on the forefront of espresso extraction? i would say no. if you’re brewing with a bottomless portafilter i.e. no splitter, are you pulling a ‘double shot’ or are you just making a shot of espresso?
i vote for ‘just making a shot of espresso’.
but in keeping with traditional italian terminology i would say a ‘normale’ (normale meaning basic, not ristretto or lungo) single shot is a 7g dose with a beverage weight of around 25g within whatever time that produces a tasty extraction / solubles yield. (let’s say around 25 seconds) a basic double shot is something close to a 14g dose and a beverage yield of around 50g, in an amount of time that (with the same coffee) produces a very similar solubles yield as the single shot (similar percentage of your dose dissolved into the cup) while a triple shot would be nearer to a 21g dose and 60g yield.
a ristretto would be any of those recipes with the final beverage made denser by decreasing the amount of water used. maybe anything noticably smaller and denser could be considered ristretto, but it sounds like the italians considered a ristretto any of the above recipes with the final beverage mass cut in half. however! a traditional ristretto is not accomplished by simply stopping the shot early. you are still attempting to reach a similar solubles yield for a balanced extraction, but with less water used, so one must fine the grind. so say your original normale shot uses an 18g dose and makes a beverage of 50g in 30 seconds, which gives you a solubles yield of 21% that happens to be the tasty extraction balance for your particular coffee. your traditional ristretto of that would be an 18g dose and 25g beverage, but if you simply stopped the shot half way to get your 25g beverage, it would only have extracted something like half the amount of coffee, leaving you with a short, strong, but severly underextracted tasting shot. so you fine up the grind, increasing surface area, and try to get a fully extracted shot (something close to 21% for that particular coffee) with half the water. however long this new ristretto shot takes to pull is how long it takes, but it will surely be more than half the time of the normale shot since the finer grind is restricting the water flow. you would have to taste test it for balance or measure your extraction using a refractometer to decide how much to fine the grind.
a traditional lungo (long) would be a single, double or triple normale with the water in the final beverage doubled. again, you want to still be reaching a similar solubles yield, so you must make the grind coarser to avoid over extraction of the dose. a lungo is aiming for more water in the cup, but about the same amount of coffee solids.
as far as splitting a shot, traditionally no, that doesn’t produce two single shots. if you dose 14g and pull to a yield of 50g but have split the beverage into two 25g beverages, each of those would be half of a double shot.
i would like to push for dropping traditional espresso terminology. some more recent trends have been closer to a 20g dose and 30g beverage yield, with many of us still calling it a double shot. but really, what’s double about it? according to italian tradition, it would be a lot closer to a triple ristretto shot. i would just call it a 30 gram shot of espresso. if you want to be more specific, it’s a 30g espresso brewed at a 2:3 ratio. or to be even more specific you could call it 20g in, 30g out, at x number of seconds.