No, it does not have to be a powered hub if all the HDD's you are going to plug in are self-powered and will not be relying on the Wii's USB port's power.
YMMV, as you can use both ports on the back of the Wii but there are compatibility limits that exist because of certain USB device, cIOS, and homebrew/loader configurations. For the specifics it will depend on the specifics of the USB device, cIOS, and homebrew/loader you use.
iv tryed it with a really cheap hub and plugged in a 16gb flash drive and 1tb HDD (both work on their own) and plugged the hub into the good usb port (at the side of the wii, not the centre) but the wii only read the flash drive, it couldnt see the HDD.
i didnt switch the HDD to another socket on the hub so it may just be the port thats at fault...