What’s the most common (and low cost) microprocessor with at least two threads? I want to make a next version of my current project. I’m hoping to use an Arduino compatible processor that also supports at least 2 processes simultaneously without using pro threading or any other software workaround. The more commonly used the processor is, the better the user support will be if something goes wrong.
ESP32.  Dual core 240MHz, plus an extra XLP state machine.  Cheap and easily available on eBay/Amazon/Ali Express etc.  And with full Arduino support.