I would like to be able to copy files from SD card (through card reader) to USB flash drive. All is connected through OTG USB Hub, but all the apps I found mount only one OTG device at a time. Is there any way to connect two at once?
YES - it is possible. It seems thought your phone/tablet has to be rooted. I've tried all the apps I found without root and could only connect one device at a time. Today I rooted my tablet and got Stick Mount app. Now everything works great - I can copy files between SD Card and USB Drive, which is what I wanted.
Here is the link to the app: https://play.google.com/store/apps/details?id=eu.chainfire.stickmount