How do I invoke spectro as a separate process in reaper?
That’s really more of a REAPER question than a Spectro question, but I’ll take it on!
When get to the "Add FX to track: " dialog, just right-click the effect (Spectro, in this case) and Run As > Separate Process. After that, you can add the plugin to your project and it will load as a separate process.
You can also set that default in Options > Preferences > Plug-ins > Compatibility, which will affect how all plugins load by default.