Here's a small but nice update from IG regarding a forthcoming functionality improvement.

As of today, in case you want to add a collaborator to your IG posts you will now need to do so at the point of creation flow, not after.

Well, you can on Reels, at least as of recently (check out this how-to guide from influencer marketing expert Lindsey Gamble), but for feed posts and Stories, you can't add a partner profile after publishing.
So it's not available as of yet, but the fact that Mosseri has specifically posted about this suggests that this will be made available soon, providing another way to utilize collaborative posts.

Which may offer a significant reach advantage. Collaborative posts appear in the feeds of both the creator and collaborator, reaching a larger audience, while they're also hosted on both users' profiles. Accepting accounts for a collaboration invite may also see all likes and comments on the post, offering data benefits.

It's another way to help you maximize your IG efforts, and soon you'll have even more ways to do this.

We'll keep you posted on any developments.