USB redundant connection

Hello:

I'm trying to do a dual connection to internet by means of two different 3G usb modems. I want usbserial control these modems, then I do:

modprobe usbserial vendor=0x12d1 product=0x1003 vendor=0x19d2 product=0x0001

This action only links usbserial to one usb modem (vendor=0x19d2 product=0x0001)

How i can link usbserial to both modems?

Thanks in advance.

OGA.