Visitor restrictions at hospitals across the US meant that Biel feared that Timberlake wouldn't be able to join her in the delivery room.
"There was a moment where there was nobody allowed at all, and I was really getting nervous about that situation," she shared. "But yes, he was allowed. I think if I had to be there alone, that would have been horrible. I would have been really scared."
As for the adjustment from one child to two, Biel told Shepard that the "balance of everything is very different and super hard".
"Someone said to me, 'Two is like having a thousand.' And that's exactly what it feels like," she said.
But when it comes to following in his famous parent's footsteps, Biel isn't so sure where she stands.
"My knee-jerk reaction is 'Oh God, no. Please no," she said. "But then I look at these kids and I'm like, 'Oh sh*t, they're probably going to be musical. What are we going to do? Like, not let them play the piano or not let them take a voice lesson if that's their passion?' … I don't want to be that parent to stifle a dream."
The couple also has a 6-year-old son, Silas.
The news of their second baby came as a shock to fans, as the star couple didn't confirm the child's birth until six months later when Timberlake revealed Phineas' sex and name on The Ellen DeGeneres Show.
"He's awesome and so cute. Nobody's sleeping," he said in January. "But we're thrilled. We're thrilled and couldn't be happier. Very grateful."