Several grasshopper definitions running at the same time

I know it is possible to open several Grasshopper definitions in the editor. but would it be possible to have several grasshopper definitions running at the same time? and even better to retrieve certain output results from a first open definition and re-inject them as input to a second open Grasshopper definition?

Not really as of now. You’d need multiple instances of Rhino+GH. There are several solutions of transferring data between sessions, though.