I’m buying a portable USB C hub to connect a USB C external drive to my iPad Pro while keeping the power connected at the same time. Do I need to find a hub with two USB C ports? One for the drive and one for the power?
Hubs are not generally intended to work as power distribution devices… they’re strictly for data distribution. You need either a specially designed device for this exact purpose (like a power “Y” device) or else you probably need a powered hub. I don’t know if you can use your iPad power supply to power a powered hub or not… any powered hub I’ve ever encountered used a barrel jack to power it. I do see a mention of USB-C power pass through, so perhaps my knowledge is not up to date.
This is somewhat related and has some useful advice What can you connect to the new iPad Pro with USB-C? - 9to5Mac