Are optical fibers full duplex?
Yes, it's done all the time; not "just in theory" and it's not at all "better to have two fibers" - in long-distance networks where fibers are expensive, many different wavelenghts travelling on both directions are a common, everyday state of affairs. The equipment to get them injected and separated at each end is expensive, so that sort of thing is not seen where fibers are cheap to install. Look up CWDM and DWDM (Coarse Wave Division Multiplexing and Dense Wave Division Multiplexing) for more detail.

On my network I have one set of 1310/1490 BiDi SFPs; I'd have more if I could find them cheaply enough (though they are MUCH, much cheaper than the devices above) - on the particular link that set is installed on they allow 3 links to run over 5 fibers where 7 were damaged, and only 2 links were possible with the dual-fiber SFPs (still in use on 2 pairs) - they work perfectly and only need the single fiber.