So I've been looking through some of the old JK threads on doubles. He says that a second run can be beneficial either 6-8 hours before or after the real workout for building capillaries. Additionally, you can get a "shakeout" effect from both runs to help you warm into your workout or stay loose afterwards.
So my question is, why not do both? As long as the runs are kept slow enough, is there any reason not to get the double benefit? Has anybody had any results training this way?