I would maybe start with List.zip/1 which gives you a result like:

[
  {"one", ""},
  {"", "two"},
  {"three", ""},
  ...
]

And then you Enum.map/2 over it with a custom function that checks the tuple and returns the value.

I would go with: :lists.zipwith(fn [], [v] -> [v] ; [v], [] -> [v] end, list_a, list_b).

Snap - you saved me typing…

I think it can also be done in one pass, with recursion, keeping the order of the list…

defmodule Koko do
  def run [], [], acc do
    Enum.reverse acc
  end

  def run [h1|t1], [h2|t2], acc do
    new_acc = [(if not Enum.empty?(h1), do: h1, else: h2) |acc]
    run t1, t2, new_acc
  end
end
x = [["one"], [], ["three"], ["four"], []]
y = [[], ["two"], [], [], ["five"]]
Koko.run x, y, []
[["one"], ["two"], ["three"], ["four"], ["five"]]

Update: Oh, it looks like @Eiji already provided this solution