Apple updated GarageBand and TestFlight on October 22 to support both the inner and outer displays of the iPhone Duo, and only one of them changes much.
Both apps keep their vertical toolbar and now display properly on the Duo in any orientation. That is the baseline fix, and it applies to each app equally.
GarageBand is the one that gains. It gets a wide layout that takes full advantage of the inner screen, with a bigger interface for music editing and for touch instruments such as keyboards and guitars. TestFlight, Apple's tool for distributing pre-release apps to testers, works largely the same as before. The Duo's larger display offers it little benefit. Both apps are free.
The split is predictable. Extra screen area pays off when the interface is the product: keys to press, strings to strum, a project to edit. A tool for handling beta builds has less to do with more room, so a correct layout is about all it needs.
That contrast is a useful guide to how the Duo's inner display will be judged. Apps built around touch and dense controls stand to look meaningfully better, while utilities will mostly just keep working. Whether third-party music and creative apps follow GarageBand with wide layouts of their own is the more interesting question, since those are the apps with the most to gain.