I have one of the newest A-A sets (a dealer special run of UP 949 and 951, which are E8/9s). I've never understood why Lionel's manuals don't make it clear that out of the box, these sets are designed to run correctly if both units have the same ID. You will know that from experience if you have one of the older sets, but that's not because the manual says so in clear terms; it was reported to be such on various forum postings and then eventually in a Lionel video. I generally test mine first without changing the IDs from the factory default just to be sure they function.
In my set, both of the units are powered but only one has sound and by default the other one without sound is the rear unit. The lighting in it will default to rear-facing right out of the box.
I set up mine to have the same ID. The same dealer has issued a Superbass B unit, but it is an E7 numbered to go with the A-A set. I have not run them in combo yet but I am inclined to use Norton's approach, which is simpler than a lash-up. Not sure what manual the OP has got but I found one that covers all of the E7 A-A, A-B and extra B units, whether powered or Superbass. This is a little more specific about setting up these combos to run either with separate IDs or with one. But you have to look deep into the manual to find it at page 40 of the copy linked below.
I can't help thinking that manufacturing these with the capacity for several different control methods, including Bluetooth, complicates matters for the likes of me who only wants to use Legacy.