If I'd had two dads, my life would've been radically different. My dad was in the military, so that never would've happened. His mom was also a devout Catholic, so the need he always had of living up to her expectations and the pain he had at never being her favorite child would've been much much worse ... Then again, maybe being cut off from them would've been a good thing! Hard to say.
On the other hand, I would hope that he would've been happier. His marriage to my mother has always been strained because she's a very difficult person to get along with. My sister and I used to play the 'what if' game growing up, mainly wondering if our parents were going to get divorced. We prayed for it sometimes. With someone more supportive at home, I wonder if my dad would've been home more and spent more time with us kids. Many times growing up my mother was my only parent and that was rough on me because we rarely got along. I love both my parents, but sometimes I just don't like my mother much.
I can't help but think that with a different partner in his life, my dad's life would've had less stress and not led to some of the health problems he's had. (would he have been a work-a-holic if he felt more comfortable in his own home?) We also would've probably stayed in one place instead of moving around and I wonder what that would've been like. I also think that with two dads I could've talked to at least one of them about the identity crisis I had as a teen and maybe I would have figured things out a whole lot sooner, and not shoved all that stuff so far away it took me years to find and sort out.
Without my mother, the dynamics between my siblings would've been very different. I think, maybe, that a second dad would've helped balance things out -- my mother has never understood or accepted how differently she treats all of us. Maybe I would have a closer relationship with my sister and brothers ... Who knows? I don't think it's wise to dwell on what might have been.