I don't have enough blood in my body to let both heads function at the same time. So being able to think during sex is a challenge for me.
All the same, sometimes I fantasize, sometimes the moment is enough. Usually when I fantasize, I am on the receiving end of a blowjob. If I am actively fucking or sucking, I'm too busy to think and am just lost in the moment.
If you have time to fantasize, maybe the moment isn't enough for you. So go ahead and let your mind add to it. Sex is mostly mental anyway.
As for your friend, maybe the moment is always enough for him.