The iPhone Duo unlocks with Touch ID in the side button, and Face ID is not on the phone in any form. The Mac Observer found in Apple's tech specs that the under-display camera behind the inner screen does no face authentication, which leaves the fingerprint sensor as the only biometric on the device. The button sits on the titanium frame, so the same finger works whether you are on the 5.4-inch outer display or the 7.6-inch inner one, and it accepts more than one enrolled finger.
For anyone who would rather not reach for a fingerprint, Apple's answer is the watch. AppleInsider reported that an Apple Watch which is worn and unlocked will authenticate the Duo automatically. Apple hasn't fully detailed how that works. Pre-orders open October 16 and the phone ships October 23, starting at $1,999 for 256GB.