Apple chose a Touch ID sensor for iPhone Duo since Face ID would have proven difficult. The iPhone Duo is Apple's first foldable device, which means it can be used opened or closed. Apple couldn't find a way to make Face ID work in both use cases, so it offered Touch ID instead.

The benefit of Touch ID on a foldable is that it can be authenticated no matter if the device is open or closed, or facing forward or backward. However, some users vastly prefer Face ID for the effortless authentication it can perform by simply looking at the device. Thankfully, Apple has a solution — Apple Watch.

While Apple hasn't fully detailed this feature, it seems as long as an Apple Watch is worn and unlocked, the iPhone Duo will authenticate and unlock too. Presumably, this would occur based on proximity and other factors. Someone shouldn't be able to grab your iPhone and unlock it as they move away, for example.

Touch ID will be more convenient for other tasks, like verifying a purchase or unlocking an app, however, Apple Watch may be able to be used for those actions as well. When using Apple Watch authentication on Mac, it pops up with a notification on Apple Watch with the option to double-press the side button. Relying solely on the included biometric authentication system will be much more secure and foolproof overall. However, if convenience is your goal, Apple Watch unlock may be the perfect alternative to Face ID.

Apple's implementation likely accounts for privacy and security concerns, but we won't know more until device testing or documentation is released. The iPhone Duo goes up for pre-order on October 16 and ships on October 23. It starts at $1,999 and comes in blue and white colors.