My hunch was completely the opposite

Two planets in the "habitable zone" both receiving "ingredients" from the same stream of comets/meteors/cosmic dust, both with what we believe to be environments conducive to life (liquid water, surface temperatures/atmospheric pressures that aren't like Venus..:)

In fact, I'm more surprised that life didn't start independently on both planets at roughly the same time (it could turn out that life did indeed start at the same time on both planets too, just it didn't "take hold" or "last as long" on Mars)

Alternately, an impact to one world caused an upheaval that transferred the blocks to another.

I do think the "galactic seeding" theory is more likely.