A mass walkabout by paramedics and emergency care workers will take place on December 21 and 28.
But unions have been told NHS staff must commit to respond to heart attack and stroke 999 calls, by health secretary Steve Barclay.
More than 10,000 staff across England and Wales are expected to strike over pay and conditions according to GMB.
The union said its members at nine regional ambulance services would walk out.
Crews are due to cover emergency category one incidents during the strike which includes times where a patient has stopped breathing and has no pulse.
But questions have been asked as to whether all category two conditions which can include strokes, burns or epileptic fits would be covered.
The Army has been told it may be called upon to provide emergency backup.
GMB, Unison and Unite unions are co-ordinating industrial action across England and Wales in a dispute over pay and conditions.
The December 21 strike happens a day after Royal College of Nursing (RCN) members stage a second day of strike action over pay.
“We hope negotiations reach a resolution”
A London Ambulance Service spokesperson said staff were already under significant pressure, adding “Should the industrial action go ahead, we will do everything we can to maintain lifesaving services for our sickest and most seriously injured patients. We hope negotiations reach a resolution so that this strike does not take place.”
Sara Gorton, Unison head of health, said: “Ambulance managers will be drawing up cover plans for each service. Unions will discuss those and ensure appropriate responses to emergencies on strike days.
“But instead of speculating about the consequences of a strike, Steve Barclay should be focusing all his energies on stopping action from happening.”
Mr Barclay said he was “open to talks with the trade unions” to resolve the dispute but added that if all public sector workers were given a pay rise in line with inflation it would cost £28 billion.