The Joe is a little awkward, but really quite simple.
First, place the body of the locomotive down.
Then, you'll need to take the
subtruck, and place it behind the locomotive as if adding rollingstock to a normal loco. Make sure you're clicking right behind the red arrow, not the back of the body. That will not make it as one.
From here I'd say use the consists tab to 'pick' it and make a consist, that way you don't need to fiddle with it any time you want to use it.
Cheers,
SM