Source for them working on both games at the same time?
Do you have the issues of Nintendo Power around 89-90? They had been talking about "Zelda 3" for years before it was actually released. You can also compare the Wikipedia pages between Mario World and ALttP and see several of the same key people working on both games: Takahashi Tezuka, Shigeru Miyamoto, Toshihiko Nagako.
EDIT: Actually found this quote in the wikipedia page for ALttP, "In 1988, development of a new NES Zelda began, but one year later, the project was brought to Nintendo's next console; the Super Famicom in Japan, the Super Nintendo Entertainment System in other regions."Last edited by The_Liquid_Laser - on 13 August 2020