With four Thunderbolt 3 enabled USB Type-C ports, the Apple 15" MacBook Pro (2018) has enough power to connect multiple devices including the Apple 12.9-inch iPad Pro. Given the Mac's dual role power capabilities, it can act as a power bank to charge the Apple 12.9-inch iPad Pro as well as being able to be charged itself. Note the Apple iPad Pro's and iPhone's that support USB Power Delivery can only consume and not supply power, hence the Apple 15" MacBook Pro (2018) needs to drain its own battery charge to support charging the iPad Pro. It's important to make sure the Mac has sufficient battery charge or connected to AC power prior to charging.
To see how the Apple 15" MacBook Pro (2018) charges the Apple 12.9-inch iPad Pro at 37% battery level, we first connected the Mac to AC power through its own 87W based charger. With a full battery, the Mac was then plugged in with the iPad Pro using a Apple USB-C to Lightning Cable. The Mac could be seen to provide steady state charging to the iPad Pro at about 12.2 watts (5 volts @ 2.45 amps) similar to using the iPad Pro's own 12W based charger over USB Power Delivery. This came as no surprise since all devices belong on the same Apple platform. Click here to view more details on how the Mac charged the iPad Pro.