Let's take this logic and go with it. No need to just hitch one up to the other. Let's bolt one to the other that way you don't waste time or energy swapping one for the other. One can push, one can pull. Surely you can see there's nothing different in this arrangement. Surely you can see there's nothing magical about 8 wheels being involved instead of 4. So let's agree that this system is functionally equivalent to the one you describe. How is this any way different than a single vehicle? Drive the back wheels while you regen the front. Then swap. If infinite range was this simple, I can assure you that you, and everyone on earth would already be driving such a thing."The mountain" was a metaphor. Towing the second Rivian is effectively providing the second Rivian with that 100 mile mountain to coast down (by towing it 100 miles).
That is the question that remains to be answered. Can the first Rivian, (the one doing the towing), tow another Rivian at least 100 miles on the power available to it? Reports suggest around a 50% loss towing 9,000 lbs. Not sure what the equivalent weight of the 7,000 +/-lb Rivian would be with regen also pulling against it.
In no way, would it be "practical" to do on a daily basis in the real world. The question only being, is it possible? Under the best possible circumstances (good weather, constant 30 mph speed where aero won't play much of a role, roughly level topography, etc)? Many believe it's not. I'm on the fence that it might be possible, under the right circumstances.
In matters of efficiency, the "real world" test you seem to think will be the arbiter of science validity will ALWAYS be less efficient than theory, not more.
This isn't a matter of opinion or debate no matter how many times you say so. It's just fact.
The Ford patent you speak of is to charge a low state battery from a high state battery, between vehicles. There's nothing sneaky there. No free lunch is being served.