Why are there two virtual USB network interfaces?


I have successfully got Ubuntu 16.04 up and running on a BBB using this howto: https://eewiki.net/display/linuxonarm/BeagleBone+Black#BeagleBoneBlack-Ubuntu16.04LTS
Interestingly enough I notice that I get two virtual usb network interfaces: USB0 and USB1. This is apparently used when I connect the BBB mini-USB interface to an Ubuntu PC, the Ubuntu PC detects the usb gadget and creates eth1 and eth2 interfaces and tries to obtain dhcp leases from both of them.
How do I change the BBB system so that only one virtual USB network interface gets configured?

Also, regarding bridging:

I have success in bridging bluetooth PANU devices to the eth0 port of the BBB using the brctl tool. Unfortunately, I cannot bridge the virtual USB ethernet interface to eth0 for some reason. Here is a post of my interfaces file:

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet manual
#iface eth0 inet dhcp

auto pan0
iface pan0 inet dhcp
hwaddress 10:00:00:f5:cf:0d
pre-up ifconfig eth0 down
pre-up brctl addbr pan0
pre-up brctl addif pan0 eth0
pre-up brctl addif pan0 usb0
#pre-up brctl addif pan0 usb1
pre-up ifconfig eth0 up
post-down ifconfig eth0 down
post-down brctl delif pan0 eth0
#post-down brctl delif pan0 usb1
post-down brctl delif pan0 usb0
post-down brctl delbr pan0

Note: The pan0 bridge does successfully get an ip address via the bridged eth0 interface.

My goal is to program the BBB to act as a USB-to-Ethernet adapter.