Testing FT230X USB i/f – best way

I just shorted together the data+/- pins(bad idea now that I've thought about it).

Nope. That's the standard, actually. The official USB Battery Charging v1.2 Spec is almost completely unreadable, but it does say:

A Dedicated Charging Port (DCP) is a downstream port on a device that outputs power through a USB connector, but is not capable of enumerating a downstream device. ... A DCP shall short the D+ line to the D- line.

The the current is literally limited by the charger. There is no negotiation of what the device is allowed to draw, the charger just drops its voltage when the device tries to draw more current than it can handle. (And the device then has to lower its draw in response or the charger shuts down?)

Of course, Apple has to do it their own proprietary incompatible way for iPods and iPhones:

The mysteries of Apple device charging

The nice thing about Apple is that they tell the device how much current it's allowed to draw, and it obeys.

And the phone manufacturers had their own ways before the standard, usually using the ID pin:

USB charger specifications and compatibility list

I don't know if there's a circuit that will work with all of the above, but if you find one, let me know.

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.