Parallel Efficiency
 

If I have 2 case which scenario would be more efficient to solve several cases?

Assume I have 8 CPU cores and two case to be run.

1) using all the 8 cores for solving a case and then going to next
2) use 4 cores for each case , and run both cases simultaneously

2nd case is always faster! Running two different simulations at the same time will utilize the cpu much more than the solver can do it. Parallel programming (=1st case) can never use exactly 100% computational power of each cpu because of idle times, competiting processes, memory access and so on. Whereas running two different simulations on the same cpu nearly do not suffer from this.