If you want to interrupt the currently playing voice, You can add the Stop Speaking
action before this
I have also recently exploring how to make Speech Synthesizer speak different texts texts at the same time. I want to simulate multiple characters reciting together. However, I found this doesn't work. It appears unfeasible to speak multiple voices at the same time. One voice must finish before another can begin.
But I think it might be possible to achieve it through external TTS technology.