Yeah lower boincs CPU use by the "On Multi Processors Use XX%" option

Since I have an "8 core" system, every 12.5% = 1 CPU

on 4 cores, it's 25% per cpu

So if you want 2 CPUs free for 2 GPU tasks on a 4 core system, set the value to 50% (75% for an 8 core would leave 2 free)