Reprogramming the seed/key on E42

So I’ve ran into an issue with reprogramming an E42 on GM. Is there a way to reprogram the seed/key on my second ecm to be the same as my first? GM said it cannot be done but there’s a guy that told me how to read the seed/key data off the ecm but I just don’t know how to program that seed/key data onto the new ecm. He won’t tell me how to reprogram it on the second one but just told me how to read it off the first/ original one. Does anyone know how?