You can be forced to go to the hospital by EMS or the police if you are unable to make decisions for yourself or you are severely injured and are not in your right state of mind etc.
If you're injured enough and about to bleed to death or have been shot or something severe like that then yes you can be forced to go to the hospital by EMS.
A patient can refuse treatment from EMS if they are coherent and can make decisions themselves.
If the patient is not severely injured or their injuries are not life threatening then they can refuse treatment but if the injuries are life threatening and they cannot make decisions properly then EMS can force the patient into the ambulance and force them to get treatment.
If you call an ambulance for someone else the ambulance will respond and then assess the patient.
Then if the patient is coherent and not confused etc and they refuse treatment or transport then the EMS will usually have the patient sign a waiver relieving them of any responsibility should something happen to them when they leave.
However if the patient is not coherent and is confused and can't make decisions themselves then the EMS can force the patient to go to the hospital and even restrain them to do so.
2 ambulances may be called to a scene if there's more than one patient or if they need more medical help.
Or the second ambulance can also be a first responder ambulance.
It’s also possible, if the ambulances met each other, that it was an ALS (Advanced Life Support) backup.
If a patient is picked up by a basic EMT crew, and needs more advanced care on the way to the hospital, another ambulance with a paramedic will intercept them.
The medic will get on board the other ambulance and treat the patient with medications and other treatments that the basic crew can’t provide.
Some services use a fly car, usually a small SUV, to meet the ambulance, others use another ambulance.
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.