what the power supply has to do here ? All USB ports have the same power available. he doesn't need a y-cable on Wii. (and he needs one on WiiU only for HDDs, not flashdrives or SD adapters)
Note that it could be your SD adapter which is not compatible ! Did you try to plug only the Gamecube device on USB Port 0 ?
You said it works on Wiiflow, so I'll suppose both are detected and working. so let's get to the settings.
You can use both USB Ports in USBGX, but you actually need to enable different settings.
but it's a mess to make things work !
To use USB Port1 you need either one of these :
cIOS Hermes
cIOS d2x v9 beta
cIOS d2x v10 r53 (this actually is v9 beta, renamed)
IOS 58
I would recommend using IOS58 in the loader which can see both ports, and cIOS d2x v10 r52 (NOT r53) for launching Wii games from USB Port 0.
This is totally normal
IOS 58 mess the partition order, you need to change a lot of settings manually to use more than one USB device.
IOS58 mounts USB PORT 1 first, which is therefore the first partition, and wbfs on the second partition.
Or if you are on Wii, you could use one of Hermes cIOS as default cIOS in the loader and partitions will be mounted incrementally, because if you use cIOS in the loader, it mounts USB PORT 0 first.
Based on the IOS you use for each feature (while in the loader, while loading a wii game, while loading emuNAND etc.), be sure to understand the partition mounting order.
You also need to differenciate port and partition. USB port0 and USB Port1 are the physical plugs. USB1, USB2, USB3 up to USB8 are the mounted partitions on the HDD or flashdrives.
So:
settings > loader settings > loader's IOS : 58
Game's IOS : 249 (or wherever you installed v10 r52 with the base you want to use)
reboots the loader.
settings > HDD settings > Enable dual USB port : ON
Settings > HDD settings > Install partition : This is where you choose "USB2" (the second mounted partition) where you have WBFS folder, should be usb2:/wbfs/, and in the loader it should be something like "FAT32" with the partition size. DO NOT pick the first partition, this is where you have the gamecube.
Alternatively, you could enable "Multiple partitions" option, which will check and list ALL /wbfs/ folder on ALL mounted partitions. the "install partition" is still used as default partition to install a new game (insert disc > install > the game goes to the "install partition" HDD)
Go to settings > user path > Gamecube USB folder : set USB1:/games/
NOTE: you shouldn't have to change this if you use IOS58, because USB Port 1 is the first mounted partition and therefore gamecube games are located on USB1 (first partition)
if you use cIOS in the loader, then it's will be ..... just guess ? ... usb2:/games/ from USB port 1 of course !
Go to main screen, trop menu, 4th icon. enable the sources you want to display, wii, gamecube, or both.
WARNING :
USB Ports are NOT loaded at the same time.
You don't have BOTH enabled and mounted at the same time, they are loaded one after the other.
the loader picks an USB Port, mounts the device, gets a list of games, unmounts the device, shuts down the USB port, and do it again with the second USB Port.
NEVER put resources on USB (covers, cheats, music, banner animation, loader settings, etc.) or it will crash the loader when shutting down the ports if there is an active resources being used.
ALWAYS use SD card for homebrew files !! only use USB for iso or roms.
it's still recommended to only use one port (usb port0) and not mess with dual USB, for your own sanity
Let me know if it works for you, and if it was easy to understand. I'll keep that mini guide in my wiki page (where there are already a few guides linked). a few people ask for this setup too. It'll prevent re-writing a guide each time.