Here are hypotheses why we have only one of them: https://en.wikipedia.org/wiki/Homochirality https://en.wikipedia.org/wiki/Homochirality Personally, my answer

Here are hypotheses why we have only one of them: https://en.wikipedia.org/wiki/Homochirality https://en.wikipedia.org/wiki/Homochirality Personally, my answer is that the mix of both is statistically unstable (repelling): the more one type of life, the more material the next generation of this type of life has - leading to total domination of initially randomly chosen one of them (spontaneous symmetry breaking). However, we are currently closing to being technologically able to artificially synthesize the second (mirror) type of life - this is a huge topic we should well understand before somebody eventually will just do it ...