According to Facebook, one option to consider is to add Stories placement to your Feed campaigns, as opposed to using each element - Stories and News Feed - independently, and for entirely separate campaigns.
As per Facebook:
"A recent multi-cell test in EMEA found that adding stories to feed campaigns by using placement optimization resulted in a 21% increase in brand lift for the campaigns analyzed. This indicates that stories alongside feed campaigns can help to drive increased efficiency."
Indeed, along with that increase in brand interaction, Facebook also found that Stories placement lead to a 35% increase in brand outcomes.
That 9:16 note relates to the optimal image/video sizing for vertical mobile placement, which Facebook recommends for campaigns in which you plan on using the two options. Facebook also recommends that advertisers use its automatic placements option, which will optimize your campaign across all of Facebook's various apps to maximize engagement.
Facebook further advises that advertisers can use new tools, like its recently launched Stories Templates, in order to convert their feed creative into Stories format, while Facebook's in-built Asset Customization tools can also reformat your content to fit into each different content stream.
Essentially, Facebook's looking to make it as easy as possible for advertisers to expand their campaigns into Stories also, and the research here suggests that it may well be worth considering.
You can read more about Facebook's study, and Feed/Stories ad tools, here.