Just in case anyone else is interested, I have finally been able to connect both a SiK radio receiver (for Mavlink) and a video receiver (to see what the video/still camera is viewing) to the TOWER app using a 3-port OTG hub for my Samsung Galaxy S6 phone.
The 2-port OTG hub I tried first would not work with the video receiver, but the 3-port OTG hub works just fine … 
Obviously, the electronics inside the 2-port hub is inferior to the electronics inside the 3-port hub.
This is the hub that does NOT work:
And this is the hub that DOES work:
This hub has a tiny switch on the side to select OTG or CHARGE mode.
The extra cable on both hubs with the micro USB socket is for connecting an external charger. Hence the reason I call it a 3-port hub because the fourth cable is for powering the hub devices or for charging the phone.