There are three kinds of USB ports in use (before USB Type C)- data only, charge only and both data and charge
Quoting from How USB charging works
In terms of actual current (milliamps or mA), there are three kinds of USB port dictated by the current specs: a standard downstream port, a charging downstream port, and a dedicated charging port. The first two can be found on your computer (and should be labeled as such), and the third kind applies to “dumb” wall chargers.
Tablets are known to have any of these ports. The port you have for charging is a dedicated charging port (as I suspect ) and will not allow simultaneous data transfer. This is borne out by your observation that the keyboard is getting power when connected with a 3 way cable or even USB hub (more reliable for such requirements)
You can easily verify this by connecting a flash drive through an OTG cable and transfer files. If it doesn't work (as I suspect), you have your confirmed answer
It may turn out to be a futile attempt by introducing additional hardware to meet your goal if the port itself cannot support. You may like to first validate this by seeking a confirmation from OEM, in case my reasoning current appeal to you