Eliminating attenuation in summing box
You can get most of the way to what you want with resistors, as long as you have enough spare level at the source, or gain following.

At the moment, your input resistors are small, and the load resistance is high. This means that when one computer is low impedance, it is responsible for all the attenuation of the signal, with equal input resistors, that's bout 6dB. When it goes high impedance, the output level jumps up 6dB, which you find objectionable.

If the 'wye' had more attenuation built in, say 10k resistors to replace the 470s, and a 2k instead of the 20k, you would lose a lot of level. This is why this method will only work if you have some gain in hand. The attenuation to the signal when the other computer is on is due to the 10k resistor and the 2k, about 1.7k. When the other computer goes high impedance, that attenuating resistor becomes 2k, and there is only a 1.5dB change in level.

The smaller you make the resistor that replaces the 2k, the less the level will change, but the more level you'll lose. 

What a proper summing amplifier does is make that 2k resistor position effectively a short circuit, it presents a virtual ground to the summing junction, which totally isolates one signal source from the other, and provides gain so you don't lose level. It's the better solution, but it requires amplifier and power source.