Linda organized it as two 2 day unconferences held back-to-back
Can you explain how that is different from a 4-day unconference, more concretely?
Mainly that we had two scheduling sessions, one on the morning of the first day an one on the morning of the third day. At each scheduling session, it was only possible to add activities for the upcoming two days.
At the start of unconference encouraged people to think of it as 2 day event and try to put in everything they really wanted to do the first two days. On the morning of day three, the schedule was cleared to let people add sessions about topic that where alive to them at that time.
The main reason for this design choice was to allow continued/deeper conversation. I if ideas where created during the first half, I wanted there to be space to keep talking about those ideas.
Also, some people only attended the last two days, and this set up guaranteed they would get a chance to add things to the schedule too. But that could also have been solved in other ways, so that was not a crux for my design choice.