Just tested the Test server.

Tried to get one of my trucks onto a trailer and then switch from the truck on trailer to the one pulling the trailer, to my surprise it worked.
Tried to get one cargo onto a trailer that was detached from truck, then attached the truck to trailer, cargo was still there and i was able to pack it.

A glitch i noticed whe first tried to get the truck onto the trailer is, the truck i switched from became translucent, like a ghost, not sure if this is intended or whats going on, i then switched to a third truck and the second one (wich i switched from) became trasnlucent, started going around the parking a biot, then the first one switched back from trasnlucent to "filled" again....seemed buggy to me, but unsure.