I'm guessing that cborrman's questions about the road legality of two 125W motors on another thread are telling... just make sure that the motor on the trailer doesn't outrun the motor on the bike and jacknifes, causing a multi-ebike pileup
Aha! So it's modular cargo space via rack(s) and container design on an ecargo bike then, which sounds useful for when you want to transport paragliding and fishing gear one day, a surfboard plus many tins of beans the next, and use mysterious "Project X" to move house the following day?