Best way to connect 2 USB devices to a single port
Probably you can hard wire the 2 power wires to those of the Gravis device but will need to switch the 2 data wires of each device so that only one device at  a time is selected. 

If you want a device that allows both devices active at once on the same USB cable you need to add the IC from a passive hub as well. These are cheap and commonly available and it should be a metter of "just doing it. ie cutting and soldering and suitable care mechanically.

The end result may not be very useful compared to just plugging the two devices into an existing passive hub, but you will learn things along the way.