As noted by AppleInsider, Apple’s single MagSafe Charger can provide wireless charging up to 15 watts, the company says that the MagSafe Duo (“coming soon”) is limited to either 11W or at most 14W, depending on the power adapter used.
If you have 20W USB-C power adapter, you’ll still only get up to 11W of charging on compatible devices. The iPhone 12, iPhone 12 Pro, and iPhone 12 Pro Max can in theory be wirelessly charged up to 15W. The iPhone 12 mini is limited to a maximum of 12W. What’s more, as AppleInsider reports, if you buy a 27W USB-C power adapter, you STILL won’t get the full 15W, as the charger tops out at 14W.
Since the single MagSafe Charger is only US$39 and the MagSafe Duo Charger costs $129, there seems to be little reason to buy the latter.