For the situation you mentioned, we have conducted multiple tests in multiple different environments. By our test, the cpu utilizations vary, because there are always some other system processes which may share the cpu time. However, we have not found that the cpu utilization of each process is more than that of a process running alone when multiple processes are processed concurrently. Even from your screenshots, we did not find the cpu is 100% occupied by the two processes of loading Workbook. There are some other processes also are using the cpu at the same time.
On the other hand, when opening a large file, it is reasonable that a processor is completely occupied by the process of loading workbook when there are no other processes with higher priority at that time. According to our test, for a 4 processors machine, if we use one process to load workbook from a file, the total cpu utilization is about 20-25%, if we use two processes to load two files concurrently, the total cpu utilization is about 40-50%, and when there are 4 processes at the same time, it is easy to observe 100% utilization of the cpu.
Thanks for your understanding!