What exact workouts are you stacking?
- Hard / Easy
- Hard / Medium
- Hard / Hard
Maybe specific examples.
The reason to ask is that stacking hard workouts on the same day may not yield the desired benefits. You need to Work AND Recover in order to get faster. The Work is the simulus, but the Recovery is where we actually get stronger.
The usual setup for double-days is to do the Hard workout in the morning and an Easy workout in the evening. That gives you the chance to add stress in the end of the day, but it is not overly taxing to the point of messing up the adaptation from the earlier Hard one.
Edit to add some existing discussions I found with a search.