Two NICs with static IP's, but can only tracert 1

[Guest]

My Win 2003 R2 server has two NIC cards, each with its own single static IP
address provided by my ISP.
I use a switch and a DSL modem to connect to the Internet.
One NIC has the Default Gateway IP provided by my ISP; the other card leaves
Default Gateway blank.
From outside of my gateway, I cannot tracert one of the NICs. I experiment
with changing which NIC has the default Gateway, sometimes giving both of
them the Default Gateway IP, and I get random results. That is, sometimes
after changing the Deafault Gateway addresses, I can tracert both IP
addresses, but later I come back and try again and only one works. Usually,
the one that works is the one with the Default Gateway populated. When both
have the Default Gateway populated, I still (even though tracert may work for
both at first) eventually end up with only one of them working. This is an
IIS server and I have various website domain names assigned with A Records
(at my ISP's DNS server) to to one or the other IP address, so I'd like to
get this resolved.
Any suggestions are greatly appreciated!!!