If you added two play actions to a single button they will start nearly simultaneously. However if you want them played in sequence, best practice is to merge both audio files into one single file.
Unfortunately, I'm not a specialist in js so I can't tell for sure.
Button actions in InDesign only start simultaneously. It has no integrated features to solve your situation.
We can assign clips to play in sequential or parallel manner on button click using Timing panel when button is selected but currently it is not supported to play them sequentially in exported epub. One workaround is to assign delay to second audio which is equal to play time of first audio. Delay can be assigned using Timing panel. We can play video from a navigation point but we cannot play audio from a navigation point.