But we will still know dad fucked them. Even tho if you say the story is different when you chose different character it doesn't matter. We know in what world dad fucks them. Not to mention your precious dev time split into two for nothing. Why don't you ask your patreon supporters and also people here if they prefer multiple protags or one mc. Pretty sure overwhelming majority of them would want a one well defined mc and also pretty sure most of them would want to play as the son.
Listen I understand you. There was once a popular game just like this where you can play son and dad. But that game didn't end pretty well and also even in that game majority of the players on play as the son. Because with son you have mom and sister fantasy. But the old guy has only daughter fantasy which is not sd popular as mom or sister fantasy in western adult games. But even if making the story as one mc means you only play as the dad, I would still prefer it over multiple protagonist approach. Because multiple protagonist truly and undoubtly sucks. It rarely works only if one of the protag is female and can end up with male mc. Most of the people put their selves in mc's shoes. It's not the same when you want them to put theirselves into two shoes. When they invest into a character and play as him and make relatioship, you ask them to play another character and watch that character fuck the love interest of your first character. Even if they only play one character, they already know the content of the other character fucking them is there. It's really a terrible way to do 'multiple protagonist'. Multiple protagonist is already sucks but when they share the same love interests it's even worse. So the core problem here is the 'multiple protagonist' especially if your game has harem-incest vibes. Anyways I don't do reviews and give low stars so I wanted put my opinion here as comment. Good luck with the game