By having them one after another you are doing just that,
its creating on on the left, then one on the right.
to make your events a bit cleaner you could change it to
create object on layer 1 at -100, random(0, windowHeight)
and
create object on layer 1 at windowWidth +100, random(0, windowHeight)
this way if you change your game resolution you won't have to go update it later.