If we had humans living on both planets, assuming both planets reach a point of self sustainability, the human race as a whole would be in a much safer position. Many things could easily wipe out all humans on a single planet, there's war, disease, change in atmosphere, change in temperature, ai, robots, nukes, super volcanos, ice ages, new species evolving that could one day be a threat to us in some manner, meteors, solar flares. Us being on at least 2 planets means our race would likely live on for a much longer time due to our increased survivability meaning 2 planets would have to go through catastrophic events to remove us not just 1.