Thread:NerdWithAKeyboard/@comment-27878566-20180712050907

https://gravityfalls.wikia.com/wiki/Bill_Cipher#Outside_of_Gravity_Falls

also supported here:

"In Rick and Morty's episode "Morty's Mind Blowers", where it's discovered that Rick Sanchez secretly stores Morty's memories, there are two memories labeled "Stanford" and "Bill C". It's likely that they are memories of encounters with Ford and Bill Cipher, since the two series are considered to occur in the same multiverse, and both Ford and Bill, as well as Morty, have travelled between dimensions. "

obviously the two shows have to stay separated since one is Disney and the other Adult Swim but they share the same multiverse and thus easter eggs of both shows appear now and then - they can't have Rick and Morty outright crossover because.. well.. the reason I stated above : they still exist in the same "reality", so to speak. 