Routing problem 2 cable modems on 1 PC

[Jer]

Dear all

I have a cable modem hooked up as my default gateway and running natd for 
my clients on XL0

I have another modem is I want to put on the same box on a different nic sis0

Problem is the remote gateway is the same for both IP's address due to the 
fact its the same ISP

I get messages saying that xxx is on sis0 but got reply from xl0
  xxx on xl0 etc


Any ideas

Thanks