Windows 2003 VPN Sever: Two VPN Internet Interfaces?

Our customer has a well working Windows 2003 Server which is used for VPN
Remote Access over L2TP/IPSec from Windows XP Clients. Unfortunately the
ADSL Internet connection fails from time to time, so that no VPN access at
all is possible during these failures. The customer ordered now a second
ADSL Internet connection from another provider to get VPN access even when
one ADSL connection is down. The customer put a third NIC in the RAS Server:
There are now two NIC's with Internet connection and one with the LAN
connection installed.

The problem I see now is that there should be only one default gateway entry
per server system on one NIC connected to the internet on the RAS Server. A
static route entry is not possible because the VPN clients can have any IP
source addresses.

Does anyone have an idea if it is possible to use a Windows 2003 RAS Server
with two interfaces connected to two different internet providers as a VPN
Server, and if yes, how to configure the default gateway entries? Is any
(MS) documentation about this configuration available?

Thank you all in advance for any help
Franz