It seems, that twine only supports a single active story line at once.
I would like to have multiple active story lines at once. What this could mean, is either, that you have multiple persons, which have their own stories, but sometimes they meet, and they have a single story, which splits afterwards.
Another example is, that you have multiple events, which can be played in arbitrary order, for example, after going to a city, you can do mutliple things (alternatively as by default) but do some things only once (so when the story is cyclic, you can go to the city more often, and each time you can do different things there)
I wrote such a program for stories not long ago myself, which also supports parallel stories. But you have to write the everything in text files instead of interactively editing, as in twine.