Apple's MagSafe Duo charger is not compatible with Apple's slightly older 29W adapter
The 29W power adapter by Apple was replaced by a 30W power adapter in 2018; however, it’s still likely that people have this older power brick. Note that it is incompatible with the MagSafe Duo, so you won’t be able to use it with the wireless charger to charge your iPhone 12 and your Apple Watch.
In order to benefit from MagSafe Duo’s fastest charging power of 14W, you will need to buy Apple’s 30W power adapter. The MagSafe Duo charges up to 11W with a 20W USB-C adapter, and maxes out at 14W with an adapter that’s 27W or higher.
Apple's older-gen 29W power adapter does not meet the power delivery requirements to simultaneously charge an iPhone and Apple Watch on the Duo.