Hi Beren_1,
The iPad Pro 3rd generation is the best iPad Apple has ever made, to add capabilities, future compatibility, and faster charging, Apple removed the old Lightning Connector and added the futuristic and modern USB TYPE C PORT to the 3rd gen iPad Pro. This is apple's only iPad with a USB C port, no iPhone has it and only some 💻 MacBooks have it. So the iPhone 6 uses a Lightning Connector which is NOT THE SAME AS THE IPAD PRO 3rd Gen. So now your iPhone and iPad will have different cables. I also recommend you to use the power adapter and cable included with the iPad for the iPad because the power adapter will be having more capacity than the iPhone's adapter.
SUMMARY
So you have to use the new adapter and cable for your iPad (BECAUSE IT IS DIFFERENT) and you may continue to use the older adapter and cable for the iPhone. No need to invest in new ones.
Always ready to help,
Arnav