Seriously impressive, you are a great uncle! The two fan sets drawing cool air indirectly through the same channel will be problematic I think and i think it would be worthwhile redesigning slightly. If one person is playing a demanding game and the other is idle, the idle pc is going to get no airflow due to the difference in fan speed and the working pcs fans will have to fight the directional airflow of the idle one. Maybe instead of this design (above view) /\ drawing air from the top and bottom, this /|/ offset design with a divider that draws air from the sides will offer higher performance cooling. I only say this because it would suck if all this hard work and generosity goes up in smoke due to slightly underperforming airflow