1. As I said before BZ used logic, you need Rinnegan chakra to awaken RinneSharingan just as you need Sharingan chakra to awaken Rinnegan.I might look like fools , but "How did Kaguya herself know this method ???"
1- She didn't have 2 rinnegan to learn this .
2- She didn't awaken her RinneSharingan , this way .
3- She was the first one to awaken RinneSharingan (According to what we know)
2. She awakened her eye by eating the Shinju, the Juubi is a part of the holy tree and the Rinnegan comes from her eye, therefore having Rinegan chakra combined + the Juubi's chakra = having the chakra that gave Kaguya the RinneSharingan