Damn Apple needs to include this in the box. Of course we know they never will. Sigh. According to Marco Arment on Twitter the 9.7" pro can't be charged using the 29W adapter.
https://www.macstories.net/ios/testing-apples-29w-usb-c-power-adapter-and-ipad-pro-fast-charging/
As you can see in the chart above, the results are clear: fast charging with the new 29W adapter trumps regular charging with the 12W model in every test. Here are a few more notable tidbits to further put the two adapters in perspective:
https://www.macstories.net/ios/testing-apples-29w-usb-c-power-adapter-and-ipad-pro-fast-charging/

As you can see in the chart above, the results are clear: fast charging with the new 29W adapter trumps regular charging with the 12W model in every test. Here are a few more notable tidbits to further put the two adapters in perspective:
- It'd take 1 hour and 33 minutes for the 29W adapter to charge an iPad Pro from 0% to 80%; the 12W model would need 3.5 hours.
- In 10 minutes with the screen turned off, the 29W adapter charged the iPad Pro by 9.7%; the 12W adapter charged it by 2.9%.
- When playing Oceanhorn (a graphically intensive game) at full brightness, the 29W adapter charged the iPad Pro by 5% (from 45% to 50%); the 12W one charged the device by 1.2% (from 27.2% to 28.4%).
- At full brightness while on the Home screen, the 29W adapter charged the iPad Pro by 7.5% (21.9% to 29.4%) in 10 minutes; the 12W adapter charged the iPad by 1.1% only.
- Changing the intensity of Night Shift from 50% to 100% didn't affect the 29W adapter; the 12W one was slower in charging with Night Shift at 50%.2
- I couldn't chart this, but charging with the 29W adapter while normally using the iPad (switching between apps with Split View, Picture in Picture, etc.) ranged from 4% to 8% increases in my tests.
- It takes 1 minute and 40 seconds for the 29W adapter to turn on a fully drained iPad Pro (from turned off display to Apple logo).