I believe the rule is you don't have to look for a witness "on the clock".
For example, if one were to complete a Saddlesore 1000 within an Ultimate Coast to Coast (which has a 30-day time limit), we would expect you to obtain an end witness for each ride.
As far as the start witness is concerned, if you are starting the rides at the same time, a single witness form and witness if fine.
Remember the prime directive - it is up to you to demonstrate you rode the ride you claim by submitting sufficient documentation to do so.
Ira Agins
Iron Butt Association