When specifying UnlitDoubleSided in the VRCFallback tag, backfaces are not rendered as expected. According to the documentation, Unlit and Toon should be combinable with DoubleSided, but this doesn't seem to work in practice for Unlit:

> Toon and Unlit can also be combined with Transparent, Cutout, Fade, and DoubleSided

However, ToonDoubleSided works correctly and renders both sides.

Additionally, while the documentation states:

> The toonstandard and toonstandardoutline tags are special, in that they cannot be combined

it turns out that toonstandardDoubleSided does in fact render both sides properly. In this case, the shader's behavior is desirable, so it would be better to update the documentation to reflect that toonstandardDoubleSided is valid and does work as expected.