I have a computer power supply and I'm wanting to make it a variable voltage power supply but I have to have a load on it which requires some resistance.
I have connected a 10 ohm 10 watt at 10% between ground (black wire) and 5 volts (red wire) but it overheats!
Can I add a second of equal value to fix this overheating?
No, since current will also double. Putting just 2 resistors with double the value (or 3 with triple, etc.) will help, since the total current will remain the same. Note that the same amount of power will be consumed, but it will be distributed across both resistors instead of just one.