The first time Shanks and Blackbeard were mentioned in the same chapter was during the time Crocodile was defeated by Luffy and the balance of power was in danger. The Gorosei learned about the messenger sent by Shanks to Whitebeard and they trust Shanks will not do anything hasty to disrupt the balance of the three great powers. Watch More here: https://youtu.be/Kfj3CSkUAU8 Read the Full blog here: https://thiswriteguy.com/anime/one-piece/shanks-theory/