As a man with straight hair that is usually on the shorter side I can tell you I get a distinct difference between shampoo only, and shampoo then conditioner. I get a similar effect from 2 in 1 that I just use that... I notice a benefit from 2 applications though (people will actively comment on how soft it looks and ask if I did something different), it is diminished after 2 though.

I try to only wash my hair a few times a week though when it feels too greasy (longer it gets the quicker this happens) and water isn't enough, usually don't bother with the double wash though.