Yeah it does seem they need to be winched together the whole way, and not become separate during the trip.
I tried this a few times, before it worked, the final time it was winched together the whole way.
It does seem you need to use the trucks with a short log bed, no trailers, so they both fit in the lumber mill square. Note, you don't have to meet the quota for the mill, just remain winched for the whole trip until the white square at the lumber mill.
I did this on the island map, there is a log yard not far from the lumber mill near the starting garage - very quick trip, not much mud, however there is two rivers to cross (small) you can push with the back truck if get stuck (triangle).
Load one truck, drive forward, (do on casual setting) but don't leave the 'loading square', park other truck behind first, load it. Change to first truck, winch both together (don't move before this), after that just drive the trip.
Hope that helps.