Android doesn’t like USB hubs in general and I couldn’t make it talk to two Emotiv EPOC dongles (that use hidraw module) using any hubs I have tried. But where there’s a will there’s a way: using a USB keyboard that includes hub and connecting the dongles to the keyboard, before connecting it to the phone/tablet, makes all devices (keyboard and multiple usb dongles) get recognized and properly mounted in the system as /dev/hidrawX. Sun keyboard type 7 is easy to take apart and supports 3 devices. It ain’t pretty, but doesn’t require external power and works fine.
Smartphone Brain Scanner 2 now supports using multiple headsets on a single device. This can be used for synchronized data acquisition or gaming on a single device. And for some other stuff that I should be soon able to share.