With @MrYoshiCraft 's 5000m run using this tech, here is how to reproduce it:

Activate both head starts in quick succession. I typically activate the purple one first and then the blue one, as that is what works for me. The other way around probably also works, but I cannot confirm that. Key word is quick succession. You have at most a few frames to press both of them to make it work. On mobile it's a bit tricky, but you can get it fairly consistent once you get the timing down.

I will update this thread soon with a video of me doing it on iOS + Touch Controls to show the timing window. Time saved with this trick is unknown, but it definitely has potential.

Also note that you have to completely release your finger from the purple head start button before pressing the blue one, at least on iOS this is true