dual fans only come on when engine is hot

There are two fans on the radiator.
These come on when engine heated.
When I turn air con on, both fans come on and car runs normal to me.
I believe one fan should always be on and the second should come one when air con turns on or over heated.
Auto electrician says no, fans should only come on when engine is overheated and do not need to always be on.
Does any one know which is the correct way the fans should be working