The casualties, named tonight as Irene Spencer, 78, and Patricia Pryor, aged 70, both from Camborne, Cornwall, were among a 48-strong party on a trip to see Christmas lights when the coach overturned at about 10.15pm last night.
The first police car to arrive at the scene in Townshend, near Penzance, also lost control and smashed into the coach's undercarriage while passengers were still inside.
One of the women was pronounced dead at the scene, while the other died in hospital.
Six people remain in hospital tonight - one in a serious condition, police said.
Jane Moore, who helped the walking wounded in the aftermath of the crash, praised the driver's actions.
"He managed to avoid two huge trees and go through the gap, which saved a lot of them I think," she said.
"He did fantastically. He was very shaken. There was no way he could have done anything to avoid it. It was sheet ice all the way down. It was like a mirror. We stepped out and nearly went over."
Garry Williams, operations manager of Williams Travel, which owns the coach, said passengers were "amazed" that the busy road was not gritted.
"I attended Treliske Hospital to speak to the driver and a number of passengers, all of which said that the coach was unable to stop once it had come into contact with the black ice, which then turned over on to its side and ended up in a ditch," he said.
"All were amazed that this road was not gritted."
Cornwall County Council said the route was not treated because it was classed as a "minor" road.
"With one of the largest road networks in the country to deal with, it is not possible for the council to pre-salt all the roads in the county," an authority spokesman said.
The coach driver and many of the passengers, some from a local Women's Institute (WI) branch, took refuge at the Moores' family home - yards from the crash site.
One of the dead women was a WI member, a spokeswoman for the organisation said.
Mrs Moore said not all passengers had been wearing seatbelts.
"Apparently, the driver said many times to put them on but not everyone did," she said. "Without the seatbelts, it could have been a lot worse."
The driver, thought to be in his 50s, suffered minor injuries.
Derek Smith, 48, a concrete producer, saw the police car crash into the coach.
"He probably skidded for about 100 yards," said Mr Smith. "There was a massive bang as he hit the coach. There were people shouting and screaming.
"There were people still inside. My biggest worry was that he had hit someone but he hadn't."
An investigation has been launched but police said it was clear the weather had "played a major part" in the collision.
Elsewhere, a Ryanair passenger jet with 129 people on board skidded off a runway after hitting a patch of ice.
It came to a halt on grass - 30 yards from Glasgow Prestwick Airport's perimeter fence and the A79 dual carriageway. No-one was hurt.
Domestic rail services were again hit by the wintry conditions as buses replaced trains in parts of south east England.
There were rush-hour delays between Glasgow Central and Edinburgh and Lockerbie due to signalling problems in the Carstairs area.
Eurostar operated about two-thirds of its normal service today but the company said it was "confident" there would be seats for people wanting to travel before Christmas.
British Airways said fog at Heathrow Airport meant it had had to cancel some short-haul flights but its Gatwick flights were operating normally.
EasyJet cancelled about 36 of the airline's 1,000 scheduled flights across the country today.
In Plymouth, Devon, a bus slid down an icy road in Keyham and hit 12 parked cars at about 6am.
The Highways Agency, which is responsible for gritting motorways and major trunk roads in England, said it was monitoring conditions, with continuous salt treatment on high-risk routes.
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