Supposedly about 30% of marriages feature a mistress. So the statistics suggest that more than one miner is in that situation.
I am not sure the story about the specific miner is really true or if it is, that he's the only man who ever dreamed up doing that. It sounds like the sort of thing someone would make up to liven the story of the miners.
I also think 'volunteering to stay down there', in most cultures in the world, would be unnecessary. In most of the world, the man would say so what and tell his wife to go make him dinner or get a smack on the head.
And in most countries of the world, the mistress and wife would probably exchange a few choice words, and then sigh, sit down together and agree that he treats the both of them like garbage.