Linux / dual screen / kde / X11 / SDL

Ok - I’ll be the first to realise this is a bit of a lazy question but it is a bit complicated and I’d appreciate some pointers.

I have a linux PC connect to 2 screens, on one, the primary I want to run X11 / KDE 4.xx. On the second I want to run SDL at an 8bpp 800x600 resolution. I don’t want KDE to every start up on the second screen.

I have previously use SDL as a window in X11 and realise I can make it full screen and I have previously used SDL with a dedicated
single screen. However, I don’t know how to stop X11 taking both screens and I don’t know how to address the second screen from SDL.

Any hints?