@Barty: It technically supports both, but, IIRC, since the naming scheme used by the hack is very strict, everything needs to follow it, including the file extension. So you cannot in fact do what you were thinking of, but you can mix & match supported font formats inside the same 'fake' family.
That applies to the K2/3/4 version of the hack.
On the K5, it's the first font family name that matters, so, provided it's different between the two formats, then you'll see both. If on the other hand it matches: here be dragons.