Just to expand on this a little, Mattel made the original He Man toys, and then they got Filmation to make the cartoon. Filmation got some "creator" rights to some of their cartoon-exclusive ideas, and then Filmation made She Ra, and basically all of She Ra came from Filmation's ideas and wasn't based on toys from Mattel. That means that Filmation has even more "creator" rights over She Ra than they do over He Man.
This is apparently why the Dolph Lundgren/Frank Langella movie never had Orko (because Orko was a cartoon creation), and why Dolph Lundgren never transformed into Prince Adam (because in the toys, He Man and Prince Adam were two entirely different people). Because Mattel (or I guess maybe Canon Films?) didn't want to work with Filmation. The live action movie was based on the toys, not the cartoon.
Dreamworks somewhat recently bought the remains of dead Filmation. So Dreamworks owns a bit of He Man (mostly the cartoon version), and more of She Ra. So Dreamworks got Mattel's permission and made a She Ra cartoon. Meanwhile, Mattel because interested in taking another crack at making a live action He Man movie. And then Kevin Smith stepped in to make this new He Man cartoon. It's intended to be a direct sequel to the old Filmation He Man cartoon. Kevin Smith/Mattel got permission from Dreamworks to do that.
So He Man is currently being made by Mattel, while She Ra is currently being made by Dreamworks. But both groups are playing nice with each other (it's a beneficial alliance), and crossovers are apparently possible. But they're both being made by different groups, who are doing their own thing, so crossovers are not a certainty (but there is definitely a chance).