It's not irrelevant. Virtually the only conceivable thing that could make Earth even temporarily as inhospitable to humans as Mars is today is an impact close to the one that created the Moon. For any other scenario, some places on Earth will continue to be more inhabitable to humans even during the event itself than Mars is today.

So, having humans on Earth + Mars at best only marginally improved the chances of humanity surviving long term than only having humans on Earth, by a tiny amount.