Now here's an interesting concept that I haven't thought of before: the Probability Space Hypervessel owners and the Secret Sealing Club together. It's so clever, considering they're the main duos of Outsider Worlders from both the PC-98 and Windows eras, but I never realized how well they work together until now. We need more of this!

There's a story arc in pool #7289 (Echoes in Front of the Temple) featuring them interacting in the Outside World with Kyouko. It starts at post #2039011.

There’s also some other doujins that show this kinda of thing, such as this pool, and this pool too...

And now we even have a pool for this gathering of characters in general! My prayers have been answered!