Mac Pro: Splitting network traffic based on destination

I have a Mac Pro tower with two ethernet ports. One of them is connected to the company intranet and the other to my home network. I'd like my Mac to use the former ethernet connection only for those services within the intranet; for everything else I'd like to use the latter connection (e.g. the web).

Given that the intranet addresses will fall into a specific range, how can this be done?

Update

Thanks to some huge help from bmike I've been making some progress but am still not quite there yet. Here is my original routing table (with some details omitted) showing the two default paths leading out of either Ethernet port:

default    A.B.C.D    UGSc           17        0     en0
default    E.F.G.H    UGScI          12        0     en1

The first entry is to the company network via Ethernet 1 (en0); the second to my home network via Ethernet 2 (en1).

The goal is to force all traffic not destined for a location within the company to route through Ethernet 2. My understanding thus far has led me to modify the routing tables as such:

sudo /sbin/route -n delete default A.B.C.D
sudo /sbin/route -n add -net A A.B.C.D

Giving me the following modified table:

default    E.F.G.H     UGScI          1        0     en1
A          A.B.C.D     UGSc          28        0     en0

The end result is not quite what I'd expect, however. I am still able to access all sites on the company intranet however all traffic to e.g., stackoverflow.com or anywhere else other than the intranet fails. A sampling from ping confirms this:

FooBar:~ username$ ping latimes.com
PING latimes.com (144.142.224.43): 56 data bytes
ping: sendto: No route to host

Is the default entry above not sufficient for that purpose, or am I missing something?

Update 2

Apparently adding the following got things working as I'd expect:

sudo /sbin/route -n add default E.F.G.H

Resulting in the following table:

default            E.F.G.H     UGSc            0        0     en1
default            E.F.G.H     UGScI          25        0     en1
A                  A.B.C.D     UGSc            2        0     en0

But I don't understand why the original default entry didn't suffice? The documentation for the I flag reads:

I       RTF_IFSCOPE      Route is associated with an interface scope

So how does that default entry's I status prevent its use for my stated goal? (I've asked a related question to the same effect here.)