The original file's structures is it, you have to follow it. By copy pasting you are trying to have 2 JSON roots, and JSON doesn't like that. This is described in the README, but the root object must be 1 *object* with entries called "layers" and "configurations". So inidividually your files would work, but you have to put them into 1 JSON object.
If you want Javascript to do it, there are resources only that explain how to do it, but the Javascript to run would kinda look like.
let object1 = JSON.load("foo.json");
let object2 = JSON.load("bar.json");
let olen = object1.layers.length;
object1.layers += object2.layers;
// You have to adjust configurations, which can be non trivial
// If config is simply an array
object2.configurations += object2.configurations.map((config) => config.map((x) => x + olen));
// Configs can be objects too. Look at the README.
I will lock this topic, as this seems to be a basic misunderstanding of how JSON works, which is beyond the scope of help that I'm honestly willing to give for a free plugin. Please open another topic if you still have trouble with things that are specifically about the plugin, and not documented (quite a bit better than mine tbh) stuff.