In principle, fork() works on Windows, and should in principle use multiple threads (= CPUs).

Depending on the actual work to be done or if there is resource contention, two instances of the program may still run slower, for example if both wait for the same spinning harddisk or try to access the same database or logfile.

You can test if there is resource contention by simply starting your program twice in parallel and seeing whether it takes the same time or takes longer overall.