Reality dies tonight!

-Two zany and wacky yet terrifying, powerful and psychotic reality-warping gods of chaos who are the main antagonists of their respective series.

-They usually pose as a friend, particularly with one of the main portagonists, although they hide much more sinister intentions (Bill posing as a sort-of muse and friend to Stanford while Flumpty usually poses as a friend to the protagonist of the ONAF games, even while trying to kill them).

-Both also are based off of a popular pre-existing thing (the Eye of Providence for Bill and Humpty Bumpty for Flumpty), are the leaders of a gang of other weird monsters (Bill's Henchmaniacs and Flumpty's gang), have pretty simplistic designs that mislead you into thinking they're less than they truly are (both designs are also somewhat similar, with both being a shape with arms and legs and a face, or whatever is the equivalent of a face for Bill, plastered in the middle), and like to weird and cruel things for the heck of it and their own personal enjoyment, whether it'd making abominationable fusion of human corpses or skin (as with the Eyesaur and the several living couches Bill made), which also leans into their general knack for body horror, to killing everyone they "loved" (Flumpty murdering all of his friends in Flumpty Night and Bill destroying his home dimension, which would've included his friends and family, particularly his parents), that and just being all-around mass murderers.

-Both are absent in their series at first (specifically the OG comics and ONAF 3 for Flumpty) and even after being introduced didn't seem to serve much of a role (Bill just seeming like a one-off antagonist like so many others and Flumpty was just a side-character introduced out of the blue in the OG comics) but were later revealed to play bigger roles and took center-stage as the main antagonist (Bill becoming his series' main antagonist and was revealed to be planning to enter to our world while Flumpty became the protagonist-antagonist of the ONAF series and was revealed to be the reason for all of the plotholes in the comics thanks to his plot immunity).

- In the final installment and climax of their series (Gravity Falls Season 2 and ONAF 3), both decided to go all out and became the final threat after the ones before them.

-Both have been noted to be recorded throughout ancient human history, having come into direct contact with us, although this shouldn't be much of a surprise since they are far older than the Universe itself.

-They also are pretty much responsible for the (although questionably canon in Flumpty's case) apocalypse of their world just 'cuz (Bill starting Weirdmageddon and Flumpty causing the apocalypse which also might be the same apocalypse as the one in the bad ending of ONAF3 via chucking a meteor at Earth for... some reason).

-Both also have a human form that never appears in the media they are in, with both of their forms being drawn by their creators for the hell of it, making them the official human versions of them. Both human forms also look much weirder than the usual "skinny young attractive guy" depiction for them.

-Both are memed off of because of a food that they either are or resemble, with Flumpty being an egg and there being several egg-related joked within the series itself, as well as among fans, and Bill is often compared to a dorito due to being a triangle.

-Both have a really weird thing when it comes to eyes, which are pretty prevelant (Bill having a giant, singular eye, something which the series alludes to and references many times, or at least something along those words, and Flumpty having an appetite for eyes, something alluded to in the game over screen for every ONAF game, and there's also the fact one of his creations is called the EYEsaur and an alternate form of his is a giant eyeball).

Not a connection I'd say is solid, buuut

Both kinda made a comback in 2021, although this is more of a fun little detail than anything else (Bill appearing as a cameo in the Simpsons and Flumpty finally getting ONAF 3 released).