You don't need to do anything special on your end except make sure that the
customer can log in concurrently. The problem the customer is running into
is on their end--they don't have Multilink configured in Dial-Up Networking.
It's been a long time since I've done this, so I don't remember any
specifics, but you have to add the second device (modem) to the list of
devices to use for the MP connection. By default, it only shows one of
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