TrainAsONE is not currently able to schedule double-run days (i.e. two workouts on the same day) - but it is on the roadmap. There are actually two parts to this:
1. Allowing users to manually initiate a double-run day. For example by creating two custom workouts for a specific day, or 'splitting' a TrainAsONE generated workout;
2. Providing TrainAsONE with the knowledge to automatically schedule double workout days. The issue here is that the science and evidence behind double-workout (let alone run) days is quite sparse. And whilst some theoretically beneficial physiological changes have been reported, (as far as we are aware) no observed performance improvements have been documented.
In the meantime, particularly if you are pushed for time, you are free to split a scheduled run into two sections. We most commonly hear of this from users who have a long run scheduled during the week who find it most appropriate to split it into two, and use it for running to / from work - i.e. run-commute.