Unfortunately is not like you are saying.

If run 1 instance -t12 ..I go on details and affinity and select only cpu 0 (I've windows 10).

in this case the speed decrease to 20H/s per cpu and total cpu at 50%.


If I run another instance -t12 and I set affinity to cpu 1, reach 100% of total cpu usage but speed decrease at 10H/s per cpu

Resuming

-t24 produce 480H/s   CPU100%

-t12 Produce 240H/s CPU 50%

2 X -t12 produce 240H/s CPU100%

-t6 produce 240H/s CPU 25%

It could be that cpuminer is not showing correclty the total speed ? because there is no sense in these results.