The two Forza studios could be collaborating on something else entirely.
Playground Games and Turn 10 Studios have been working in synergy over these past few years, with both developers co-managing the larger Forza franchise, through Horizon and Motorsport respectively. While Turn 10 Studios are working on the next Forza Motorsport game, Playground Games have moved on to some completely new, with a reboot of Fable.
However, it seems Playground might still be working with Turn 10, receiving support from the studio for Fable’s development to at least some extent. As pointed out by VGC, the LinkedIn profile of Bill Geise, Design Architect at Turn 10 Studios, mentions that he is currently working as Design Architect on Forza and Fable.
Geise has been at Turn 10 Studios since 2004, and has been involved in the development of every single Forza Motorsport game, including having been the creative director of the previous three.
It is, of course, entirely possible that Microsoft first party studios are working together on an in-house Microsoft project- though Turn 10’s involvement in Fable isn’t something we have heard anything official about so far.
Fable and Forza Motorsport are both currently in development for Xbox Series X and PC. Neither of them have release dates yet.