Depends entirely on the dynamic of the two people since backend and frontend has to work well together. When you're in a early startup, you want everyone to be on the same page, so you want people to collaborate closely. If they are people who like to work in silos without talking with each other, you'll have bunch of impedance mismatches at that stage. So you'll want a close team early on and if you can find someone who is good with both frontend and backend and could build it as one person, that'll be better for the early stages. Or two people who can collaborate closely.