(Firstly, what I mean by a second life is living with another husband and kids etc) I think my dad is going through psychosis because all he does every day is sit on the computer and try and find answers to things that aren't true, there's nothing I can say as his son to try and make him believe different because it's what he believes and he refuses to change his mindset, he's constantly making my mother cry and she's tried to commit suicide because of it, I'm terribly stressed and I don't know how to help, he refuses to see any more doctors and take his medication, every time he asks for the truth from my mother, when she does tell the truth which is she has never had an affair or spy on him, it's always turned around saying that she's lying, I don't know what to say about this anymore, it's consumed our lives and my siblings lives, if there's any tips to get my father back on track that would be greatly appreciated, thank you.
I would suggest that you contact Adult Protective Services (it might have a slightly different name where you live). File a report. They should go out to your parents' house to do a welfare check although I'm not sure what their criteria for this is. But it's easier than getting either of them to the doctor or admitted to a hospital and it brings in the authorities.
You might accompany your mom to a doctor's appointment and ask for their family doctors advice.
You could help your mom make an appointment with a psychiatrist for your parents to get " marriage counseling" so that an MD can see your father's delusions in person.
Don't try to convince your dad of anything. Can mom leave and live elsewhere until he gets treatment?
How old is your father?
How long has this been going on?
Was everything fine before this?
Your father needs psychiatric assessment. There are ways to make this happen which better informed posters than I will be able to explain - only it's the middle of the night in the USA so many of them may not be around for a few hours.
Reassure your mother and send her comforting thoughts. There will be a way past this, and I'm sorry for what she's going through.