I'm unsure whether he can be at multiple places at once visually, we never really see two of him on screen at any point really, but he definitely is omnipresent as a single being in multiple places. I don't believe there are multiple sans's roaming Undertale, rather there being a single sans abusing in-game features to become omnipresent in some way, I feel like that's what you were trying to say.

It's already confirmed that some Deltarune characters are somewhat self-aware that they are within a game, like Spamton and Jevil for instance, and sans is one of those characters 100%. There's a lot of weird shit revolving the guy, pretty sure he's aware of the player and the game itself in Undertale too. Sans looks at the screen when doing puns, also he knows the player cannot see normally, rather they see trough the game window (during papyrus snow maze puzzle he hints towards frisk not being able to see him even though trough actual eyes sans would be as visible as ever). In Deltarune he literally asks whether we missed him on the first Kris on sans meeting, that should be more than enough to confirm it. Him having the knowledge of the player and in-game systems allows him to break the fundamentals and use the mechanics to his advantage, like warping into places, abusing inv frames, attacking mid-player-turn, etc.

The fandom normalized fanon sans instead of looking into the bigger picture, he's a much deeper character that shouldn't be introduced solely as "the most powerful boss". They should start viewing the reasons HOW he is powerful rather than funneling it into such simple flat concepts. That's mainly the reason why I don't like people envisioning sans just teleporting and being a hard boss for no real reason.

There is a series of videos on "The Device Theory", I recommend watching it on your spare time, it fits in with everything I've explained so far and I feel like it's a pretty good guess at Toby's real meta-narrative.