Electronic – How to easily connect to the breadboard circuit using USB

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.

As Steven said, a normal USB 2.0 bus can provide up to 500 mA at 5 V. However, a device is only guaranteed 100 mA until it negotiates with the host to draw more. Depending on how much power the host has available, it will either grant the device the ability to draw 500 mA or shut it off.

In practise, most desktop motherboards just connect the USB 5 V line to their internal 5 V power supply in series with a polyfuse. Desktops have plenty of power, so it's not worth the expense to put the extra power detection and switching into the hardware. Laptops however have limited power and they often do have the additional hardware. From a laptop you are more likely to only get the guaranteed 100 mA without active communication.

If you are using a desktop to supply the USB power and you can grab 500 mA without talking on the USB, then you get up to 2.5 Watts. Actually you get a little less because the minimum guaranteed USB voltage at full current with the maximum length and thinnest allowed cable is something like 4.3(?) Volts. In practise with a desktop and a short cable, you will get pretty close to 5 V even at 500 mA.

So what you ask it doable, but in the end I'd just get a off the shelf 5 Volt power supply. Nowadays wall-wart style switching power supplies are cheap and available. The minimum you can find will probably supply at least 1 A. That will be more tightly regulated than what you get from USB.