Impuls in double formation

[jmluengor]

I started the fifth mission of Patwrogel by selecting the LKA option, with the Impuls 2. Initially, I start with a single railcar and couple a second one to a parking track. After successful coupling and battery activation in the second vehicle, I switch cabs in the main vehicle and report it over the radio. The traffic light turns white, but I can't move forward because my vehicle only has traction in the main unit (although the second does appear on the HUD). Obviously, I understand there's something I'm doing wrong. Can you tell me what it is? Thank you for your time.