I think one day events are easier on most peoples schedules but a two day course is more valuable in that the instructors can simply continue on with a curriculum from the day before with less time spent "re-starting". Even if there are levels of instruction held on different single day events, there is some time efficiency lost as students have to be brought up to speed at subsequent events to remind them of what they have forgotten.
Maybe a saw off is basic/intro level courses on single days and then more advance curriculum over two days.