Exporting the actor or behavior would certainly do it, but that still involves closing and opening the two projects.
I don’t know if this can still be done, but when I rebuilt my game a while back, I had two copies of Stencyl open and I copied and pasted between the two of them. Worked just fine at the time. When editing a behavior, you can click it’s event name and copy/paste that between Stencyls. Just create a blank behavior to paste into on your main project.