It is not spraying once or spraying twice you should be looking at it is how much solution your using per tan.
Spraying once with 40ml to 50ml will give a nice tan with nice fade off.
Spraying once with 100ml will give you a tan that will fade badly and possibly have crocodile/snake skin efffect as you have overloaded the skin with DHA (by applying too much solution).
Spraying twice with 40ml to 50ml (by having the gun turned down to a lower setting) will give you the same lovely tan that wears off well, as spraying once with 40ml to 50ml would do.
So it's not how many times you spray but how much solution you apply to the body that matters.
Learn how to set your gun to suit your style, if you prefer to spray twice then set your gun lower, if you prefer to spray once then set it higher BUT not too high. Do the couch roll test to see how much solution is coming out, spray 1 swipe down the couch roll as you would spray a person, you want the colour to show on the front of the couch roll but the back of the couch roll to be dry and colour free.