Depends if they are homophobic or accepting. Before coming out (as demisexual and grey panromantic), I knew my dad was (still is) homophobic but I wasn’t sure about my mom. I had a feeling she’d be more accepting than him though, and I was right. She actually knew before I knew.
If you know one parent may be more accepting than the other, come out to both simultaneously. I wish I had that opportunity (parents don’t live together), and if I didn’t come out separately, perhaps my dad wouldn’t have rejected me.
If you have the opportunity to come out to both parents simultaneously, take it. That’s only if both parents aren’t homophobic. If they are, then maybe you shouldn’t come out. Someone who isn’t homophobic should be in the room so they can influence the others, in short.