I've thought about building one with my spare pack from my EV but range anxiety is something you get before you buy the car, not after you've owned it, unless you can't do math and bought a vehicle that doesn't fit your needs.
If you have two packs you want to use them both at the same time because you will get more range out of them as you discharge them both slower.
If you look at
Steve Hawkins' RXT-B (range extending trailer) you will see a battery trailer that is adding about 55 miles of range, the guy says he's increased his range from 80 to 135 miles by adding more or less an identical pack to what the vehicle that is towing it is using.