you can only use one usb device at a time.
this is from wiimc website..
USB
USB Mass Storage devices are supported; however, several things should be noted:
FAT, NTFS, and ext2/3/4 filesystems are supported.
Although you can use either USB port on the Wii, only one USB device (ie: one port!) will be visible at a time. It is not possible to change the device that appears – you will have to unplug one to see the other. This is not a limitation of WiiMC, but rather the library it uses, libogc.
Although the majority of devices work, not all devices are supported. This is not a limitation of WiiMC, but rather the library it uses, libogc.
IOS58 is required for USB 2.0 support and for greater device compatibility.
Assuming your USB device is visible in WiiMC from the file browser, just copy media files to the USB device and load them.
i just made a folder on usb loader harddrive and copied films onto that...