We had the same problem in our game! We wanted to plug multiple AudioStreamInteractive into an AudioStreamSynchronized (one for each "layer" to switch between the different layer-states), but ended up using a AudioStreamSynchronized and just hoped we stayed under the sample limit.
The Problem with AudioStreamInteractive is that as far as I can see you can only switch from a) the player via the property (if the stream is directly being used) or by calling switch_to_clip() on AudioStreamPlaybackInteractive, but you can't get the Playback object from an AudioStreamPlaybackSynchronized. I wrote a godot engine proposal + a patch for it (after the gamejam), so I hope that either gets merged or someone comes around with a much better solution than implementing a get_playback() on AudioStreamPlaybackSynchronized. ;)