If nothing else, you can register several accounts, and open several games in different windows. I don't know whether that will set off flags, though, and it would be less convenient than something specifically set up for simuls.
As of 29 April 2020, Chess.com supports playing online simuls (simultaneous exhibitions) against human opponents.
To enable, users must select the setting "Play Multiple Games at a Time" at chess.com/settings/live.
Enabling the "Auto Switch Game" setting, "Chess.com will automatically take the simulgiver to the game in which they ...