Thanks for spotting this typo! There should definitely be a to before start.
As for whether it should be to start or to start on, I think both work. Let’s try this:
When you want to start on two things at once, you can’t avoid the need for diverging control flow.
It fixes the mistake, but keeps the slightly more informal tone. Let me know if you still find the sentence confusing, though! If so, I’ll rethink.