Ok, just read that, did tests with 1TB FAT32 USB HDD (my Wii HDD) in top back USB port, and 4GB Wii U Formatted USB stick in left front port. Wii U has system version 2.1.0E.
First test, with both in, switched on Wii U, and got same result as FIX94, Wii U Menu complains about not supporting two, and to turn off console and remove one, and trying to select OK and ignore message, Wii U Menu reloads, and complains again.
Second test, unplugged Wii HDD, Wii U Menu loaded fine without complaining. Load up WiiMode, and loaded HBC. HBC tried to read USB stick, then plugged in USB HDD, then pressed 1 to bring up device menu, tried selecting USB device, HBC crashed, code dump. Trying again, No USB devices connected, loaded up WiiMode, loaded up HBC. Connected USB HDD, HBC sees it fine, loaded USB Loader GX fine. But mine does not need 2 USB ports, it is powered by wall power outlet.
Then went back to Wii Menu, plugged in USB stick again, with both plugged in loaded HBC, HBC tried reading USB stick first instead of HDD, unplugged it, and then recognized HDD fine again.
So it looks like Wii U does not like having more then one USB device at once, especially if one is Wii U format and one is not.