I have a device that exports two interfaces, one mass storage, and one
'custom'. I'd like to use the ugen driver to access the 'custom'
interface, but I can't seem to figure out how to configure a kernel to
do so.

Apparently, whichever USB driver claims an interface on a device
claims the entire device. If I configure umass into the kernel, and an
ordinary ugen, then the entire device gets claimed by umass.  If I
configure ugen to recognize the vendor/product with 'flags 1', then
ugen claims the entire device.

From the spec, and from the way other OSes interact with the device, I
believe this to be a bug. I think that each interface should be
connected to a different driver in this case, especially since the
interfaces claims to be different classes.

Is this a bug, or am I not configuring my kernels correctly?