Offscreen rendering & output on multiple monitors.

I want to render (realtime) to a big offscreen texture, and display parts of that texture in different windows / on different monitors, while controlling parameters from a UI.

My idea :
- Main thread runs the Windows UI,
- spawns a Render-thread that renders stuff to the offscreen texture in the background,
- and creates a couple of output windows, windowed or optionally full-screen, on different monitors.
but how do I handle the "output parts of the rendered texture to multiple monitors" part ?

Each of the output-windows would get it's own render-thread I assume,
and in that I'd bind the offscreen rendered texture to render a quad in the window.

Can anyone help me a bit more on how to make this all work ?
I need to share contexts I think ?
How do I sync the output-window display with the background-render thread ?
General advice ? :)

Cheers !