Dr Anthony Marsh, the head of the West Midlands and East of England services, is paid a total of £232,000 a year.
Health Minister and Suffolk MP Dr Dan Poulter said he intended to ask the chairs of both trusts to "explain what they're doing with public money".
An ambulance service spokesman said: "It's ludicrous to suggest the taxpayer is getting poor value for money."
Dr Marsh previously worked five days a week for the West Midlands service, but also took on the role with the East of England Ambulance Service Trust in January.
The combined role brought a salary increase of £50,000 but meant an overall saving of £130,000 by not having two substantive chief executives, the East of England Ambulance Service (EEAST) said.
However, Dr Poulter, the MP for Central Suffolk and North Ipswich, described the salary package as "unacceptably high" and said it sent a "very bad message" to front-line staff.
"Effectively Dr Marsh is being paid two salaries at the same time, even though he's only doing half a job for each ambulance service.
"It's obscenely high and a profligate waste of money."
Dr Poulter said he had raised the issue with ministerial colleague Earl Howe, who is responsible for the ambulance service within the Department of Health.
"There's no doubting that Anthony Marsh is a very skilled man with a lot to offer to the ambulance service... but when you're in a leadership role you have to set an example," Dr Poulter said.
"I'm going to be raising the issue directly with the chairs of the ambulance trusts and calling them in for a meeting to make sure they can explain what they're doing with public money, and explain why they're not prioritising front-line patient care with the money they're given by the NHS and then by the government."
A EEAST spokesperson said: "Since his appointment, Mr Marsh has saved taxpayers approximately £8m of back office functions and management savings to help fund the recruitment of 400 new staff, with more than 120 already having joined the trust, and brought in 147 brand new ambulances and more than 60 rapid response vehicles.
"The organisation has turned a corner thanks to the actions Mr Marsh has put in place."
The salary of the chief executive of two ambulance services has been branded "obscene" by a government minister.