The bodies of Alexander Forman, 32 and Robert Pooley, 36, were recovered from the four metre deep pit at Newlands Farm, Sunk Island, on December 14, 2015.
An inquest in Hull heard that the men had tried to clear a blocked pipe around 9am that day, with only partial success.
Stockman Paul Loftus told how he had descended a ladder into the one-metre square reception pit to try and clear the obstruction, probably plastic, from the "toys" pigs are given to chew, but had suffered no ill effects.
Asked whether he had been given any warnings about the dangers of going into a slurry pit, which can be oxygen deficient and full of noxious gases, he replied: "I wouldn't have been down there if I knew that would I?"
The two men were found around 4.30pm when the pit was full of slurry around a metre from the top. Mr Forman, whose body was recovered first, had a leg stuck in a ladder rung.
The inquest heard it was impossible to say who went in first.
Pathologist Dr Richard Shepherd said the men would have quickly lost consciousness, due to lack of oxygen.
He said: "Unconsciousness follows and the individual is unable to protect themselves, but is unaware of the events that then follow."
The cause of death was given as immersion in slurry.
Mr Forman, the farm manager, was part of a partnership with his parents Susan and Robert Forman, based at Newlands Farm, which finished pigs on contract for Yorkwold Pigpro Ltd.
He was described as a "doting Dad" who was excited about seeing his young son Felix in a reindeer outfit for Christmas.
His partner Lucy Hartlebury-Forman said when Mr Forman senior came to the door and laid his head on his arms on the table that afternoon, saying: "Oh no, oh no", she thought it was part of an elaborate plot to get her to come to the farm so his son could propose.
It was only when a fireman pulled a body out of the pit, the terrible truth dawned: "I saw a silver bangle on his wrist. It was exactly the same as I'd bought Alex, it was then I realised it was Alex."
Mr Forman's mother Susan described the events as a "living nightmare" and said both men were experienced farmers who knew about the dangers of slurry.
She added: "They had placed a ladder on the inside to go down. I can't think of any reason why they would do this.
"It is a big no-no which has ultimately cost them their lives."
Mr Pooley's partner Amanda Pooley said the father-of-one was extremely hard-working and "loved being a daddy."
She said if there was a job needed doing "he would just get on and do it." She said in her statement: "I can't believe that he knew the dangers of the slurry pit as in a life and death situation.
"I truly believe he would not have gone into it, knowing it could cost him his life."
The hearing was told that no charges have been bought against anyone in connection with the double tragedy, but a Health and Safety Executive investigation is continuing.
HSE inspector Sarah Lee said Mr Loftus may have been "an extremely lucky individual."
Coroner Prof Paul Marks said at the conclusion of today's evidence: "This is possibly one of the most unpleasant cases I have heard for a long time.
"I feel sorry for the families having to listen to such distressing evidence."
The inquest continues.
Why does slurry cause problems?
When slurry is broken down by bacteria it produces methane, hydrogen sulphide, ammonia and carbon dioxide.
Hydrogen sulphide gas is poisonous to humans and animals.
As Coroner Prof Paul Marks put it it creates "an unpleasant combination of reduced atmosphere which is deficient in oxygen."
He said: "It is really when your nose and mouth becomes level with the miasma of gases that you run into trouble."
The only people who should enter a slurry pit, Ms Lee said were “specialist contractors” or “competent staff” equipped with “specialist safety equipment.”