The way you’ve worded it, I imagine 2 scenarios of what you want to do.
a.) You want the editor in the external monitor and the viewer fullscreen in the original one.
In that case it’s simple. Load the editor or move it to screen 1, open the files you want to work with, go to Settings > External Monitor > and switch from “None” to “Screen 0”. Basically just the reverse of what you’d do if you’re setting up a normal environment for an external monitor.
b.) You want the viewer in fullscreen on the same monitor the editor is in, regardless of using an external monitor or not. (And also not windowless-fullscreen)
I haven’t found a way to do this either. Scenario A could be considered a workaround if this is what you were looking for. Sorry.
And if neither of these are what you needed, could you be more specific please?