Hela: of Mice & Magic supports up to four players online and up to two players in local split-screen. The two modes are not the same feature at two sizes: online sessions are the full four-player configuration, while split-screen caps at two people sharing one display, two per screen at most.
Solo play is not a reduced version. The Shade System conjures magical copies of your mouse, so a puzzle that expects four paws can still be solved by one player, and the campaign stays open from start to finish without another person.
The demo is the exception
The free demo that closes on 9 November supports local split-screen only. Online co-op is not available in it at all, which has caught out a fair number of people who downloaded it expecting to play with a friend in another city. Online play arrives with the retail build on 1 December.
One quirk worth knowing before you start: in the demo, the second player has to join before the session begins. Dropping in halfway through does not work.