The reason they send 2 ambulances is because one is usually a first responder ambulance local to you and the other ambulance is the transport ambulance that takes you to the hospital.
When you call an ambulance usually a first responder ambulance will respond.
The first responder ambulance is closest to your location and will respond first and then if needed the other ambulance that takes you to the hospital will show up.
In some cases you may not even need to be transported to the hospital and treatment can be done at your home or location by the first responding ambulance.
However if you do need to go to the hospital then the first responder will request the second ambulance to come to you to take you to the hospital.
Even firefighters may show up before an ambulance if they are closest to your location.
Firefighters are also trained EMT and can render medical aid to you while they wait on the ambulance to come and transport you to the hospital.
So in some cases a firefighter may show up or a first responder may show up in their personal vehicle or even the police and then the ambulance depending on who is closest to you.