USB drivers

The last time I programed was over 15 years ago and I was thinking of 
looking into the USB drivers for FreeBSD concerning multifunction 
devices. If I ask stupid questions forgive me because I am seeing this 
as a user.

First I would like to know if there is a technical reason why two 
different divers cannot attach to the same device. If each driver works 
well independently, what is the reason to prevent the second one from 
attaching? I would assume that if you attempted to use both drivers at 
the same time you would get a device busy error.

Am I misunderstanding something here? I see ulpt: and unlpt: attaching 
to the same device and the choice of which one to use is up to the user 
and the hardware they have. If we have two drivers attach in one case 
why, and where, do we prevent other drivers from attaching if they meet 
the minimum requirements?

Any help on how to write drivers for FreeBSD would be appreciated.

Chris