Late model iBook G3 support spanning (as oppsoed to mirroring) via the standard spanning hack (which was first developed for these). I believe that the models from the 800Mhz iBook support the hack (I certainly remember having an iBook 800 and it was supported but the previous generations were not).
If you iBook is too old for the spanning hack (and I'm sure the one in your sig is too old) then I don't know of any other solutions.
Edit to add: The original hack says you need a Radeon Mobility with 16Mb VRAM which started with the 600/700Mhz generation.
yeah, you can mirror obviously, but don't try the spanning hack, because at the best it won't work, at the worst, it'll fry your video card, and then you won't be seeing anything.... 🙄