If a loco has two cabs, then to change cabs switch to internal camera, press CTRL/C, and if desired switch to external camera. This will not change control to a different loco.
If a train has a powered rolling stock vehicle (a loco, etc) on each end, then to change control to the other end switch to external camera, select the loco which is to have control either by clicking on it or by pressing the + or - keys on the main keyboard, and if desired switch to internal camera. Don't press CTRL/C unless the selected loco has two cabs and you wish to select the other cab.
AFAIK these operations are the same in DCC and CAB modes.
HTH, John