Linux driver for multifunction USB device

Hi,
I am writing a driver for a composite device with two interfaces.One
interface is a modem interface and the other is a diagnostic interface
to be used by diagnostic applications for the modem.My queries are as
follows

1.Do i need to write two separate driver modules for the two
interfaces or a single driver would do the work ?

2.How does the user applications talk to the modem driver module?

3.Do i need to write a driver module which enumerates the two
interfaces??

Any help in this regard is highly appreciated.

Thanks in advace..

Sonu Abraham