It will work. You are going to be able to charge both your iPhone, AirPods (if you have a wireless charging case) and your Apple Watch using the Apple MagSafe Duo combined with the Apple 29W USB-C charger.
The 29W USB-C charger lacks the necessary USB-C Power State required for 15W charging. When charging an iPhone 12 / 13 using your MagSafe Duo you are going to be limited to 11W instead of 15W.
Charging an iPhone 12 or iPhone 13 will be slightly slower, but it will work. You should also consider that tests using different chargers connected to the MagSafe Duo or the MagSafe Cable / Puck for that matter shows very small differences between running it at 7.5W, 11W and 15W. The faster charging is only happening if the battery is at very low levels (sub 50% it seems), after you reach 50% the remaining time required to get you from 50-100% is almost neglectable regardless of you using a charger being able of using 7.5W, 11W or 15W.