Routing issue

[Web Walrus (Robert Wall)]

I just installed a secondary internet connection at my office, and I'm
having a bizarre issue...

I have a network card - dc0

That network card has a config roughly like

ifconfig_dc0 inet 1.2.3.4 netmask 255.255.255.248
ifconfig_dc0_alias0 inet 2.3.4.5 netmask 255.255.255.248
defaultrouter="1.2.3.1"

When I have the network set up in this manner (packets coming in via two
external lines plugged into the same switch), I can only access the
network that is on the same network as the default router.  In the example
above, I can access the server by 1.2.3.4, but not by 2.3.4.5.  If I
change the defaultrouter to 2.3.4.1, I can access the server by 2.3.4.5
but not 1.2.3.4.

The ultimate goal of this, obviously, is to enable both interfaces to work
from the outside world.  Traffic needs to be able to come in either/or,
and leave by the external device of my choosing.

Is there something I should be checking, or something that I'm
overlooking?

Any help you could give would be *greatly* appreciated.  Thanks!