For those wondering, it's actually quite common that two spacecraft launched that way (if two spacecraft are launched, which is not common) would arrive out of order. The reason is that you can think of a "launch window" as really a specific ideal launch time, but a little earlier or later will also work, although being early or later would result in arriving at the destination later. They aim to launch the first spacecraft right at the opening of the window, so if there is any delay they won't run out of time. If it launches "on time", then it actually launched early for the launch window, and takes longer to get there. The second launch right after then launches closer to the actual ideal launch date.