I interpret it as drawing a distinction between how we live before vs after coming to terms with the fact that our time is limited.

When you are young you live like you don't care about your future. At some point you realise that you have only one body and that certain things will only get worse, so then you start caring and living more carefully.

Think about it like a mid-life crisis, or some other point in time where people say "What am I doing with my life?" and then decide (and have the means) to make a change. Now the second life has begun.

For me I changed at some point when I really realized I will die at some point (and so will my parents). You have "two lives" during your one life, and you realize this precisely when you realize you have but one life.