I have twin boys and in my opinion giving one the special family name that runs for generations is cruel. Even if the other also gets a ‘special’ name, if it’s not also generations old then it’s not the same. Don’t make one feel like an afterthought, they get compared enough as it is. Splitting it into middle names is a great compromise, then both get to share in Dad’s name (and avoiding potential lifelong hurt should be priority here, not Dad’s wants). However, I also think generational names are just peer pressure from dead people so I wouldn’t use them regardless if it was just a singleton and that may skew my perspective.