If you have one dual core CPU then for sure you could run two workers without them interfering with each other.
If you have a CPU that supports hyperthreading and it is enabled in the BIOS then you could run two workers per core. However, the technology involved is essentially that the second thread per core only gets to use the CPU when the other thread does not need to compute, such as if it is handling an interrupt or waiting for something to happen. You do not get to do two things simultaneously, you just get faster switching when one starts to slack off. This might be fine for workers that are reading files, but not if you have heavy computation.
You can request any number of workers in your pool, but beyond the (number of cores) times (number of active hyperthreading per core) you would definitely be into the slower operating system work management where workers compete with each other for access to a CPU core.
For heavy computation, using hyperthreading is often not very useful.
Note: on an active system it can be effective to deliberately leave one core unused for the operating system to interact with the user and run virus scans and other similar overhead.
