Add /dev/fb1 to seat1, start kwin with the framebuffer backend on seat1, verify with the stdio/stderr output it selected /dev/fb1.
Make sure manual device selection still works
There is an issue right now where /dev/fb1 is all black, if it is both selected manually or automatically. I think I might need see if the VSCREENINFO or FSCREENINFO framebuffer settings are changing or something (I had to do something similar to this to Weston's fbdev backend as well)
If I do find a solution, I'll send one separate from this series