I'll repeat the answer I wrote on your other post with the same question (https://community.articulate.com/discussions/building-better-courses/alternative-to-lack-of-storyline-responsiveness-vertical-slide-decks):
Storyline doesn't allow different slide sizes in the same project.
I don't think creating two separate projects is a good alternative. That'd mean extra work up front, plus extra editing work every time an update is needed.
Why not just develop 9:16? That'd be great for the 75% of users on mobile devices. And I think it'd be acceptable (albeit weird at first) for the remaining 25% on devices with 16:9 screens.
Also, you could insert 9:16 Storyline blocks into Rise when SL functionality is needed.
