I have been away from Cinder for a while, busy with other stuff, so forgive me in advance if it's not working, but I have a sample that does what you want:
At the very least, you could have a look at it for ideas. It's pretty easy to render the same scene in multiple windows, but you'll have to understand that you're going to do the same work multiple times and performance will be an issue. Make sure to draw the least amount of polygons by performing culling where possible. I don't think culling is part of the sample, but you'll find some culling code in the samples that come with Cinder. Let us here on the forum know if you have more questions.
Edit: Rich's suggestion to use proper hardware is the better way of doing things. If that proves impossible or too expensive, try rendering to an
Fbo first, then copying the correct portion to each window. The solution in the sample above should actually be your last resort. But it might be of interest nonetheless.
Problem with rendering to a single
Fbo first is that it requires more memory and, additionally, you can't share the
Fbo across each of the window's graphic contexts. However, you can share the actual texture used by the