If we can, I'd like to move away from discussing why the question was asked and instead focus on answering the question itself, please. Something is different in 10.04 and even if the purpose may not be applicable in a particular environment, the result still remains interesting, worthwhile, and, in fact, may apply in ways not otherwise immediately evident. I am really hoping there is a security protocol, kernel setting, or service running that was implemented somewhere between 8.04 and 10.04 that can be identified, if not managed.

As for proof of concept, here are the ifconfig and route values from installs where two interfaces with the same gateway works, 8.04, Debian 5.04, OS X 10.6.4, and Open Solaris 9, and a non-working 10.04. I just created a default 9.10 install and this 'problem' exists here, too. Note that Open Solaris and OS X do not have a 'route' command that returns routing like Linux, so I used netstat -nr

Working 8.04 configuration:
eth0 Link encap:Ethernet HWaddr 00:30:18:ad:ed:ec
inet addr:10.1.1.15 Bcast:10.1.1.255 Mask:255.255.255.0
inet6 addr: fe80::230:18ff:fead:edec/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:26595768 errors:0 dropped:0 overruns:0 frame:0
TX packets:12716759 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:3427873578 (3.1 GB) TX bytes:3744703590 (3.4 GB)
Interrupt:16

eth1 Link encap:Ethernet HWaddr 00:30:18:ad:ed:ed
inet addr:10.1.1.16 Bcast:10.1.1.255 Mask:255.255.255.0
inet6 addr: fe80::230:18ff:fead:eded/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:27604069 errors:0 dropped:0 overruns:0 frame:0
TX packets:54302172 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:4183963946 (3.8 GB) TX bytes:1736026744 (1.6 GB)
Interrupt:17 Base address:0x2000


Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
10.1.1.0 * 255.255.255.0 U 0 0 0 eth0
10.1.1.0 * 255.255.255.0 U 0 0 0 eth1
default blackbox.xxxx 0.0.0.0 UG 100 0 0 eth1
default blackbox.xxxx 0.0.0.0 UG 100 0 0 eth0

Debian 5.0.4
eth0 Link encap:Ethernet HWaddr 00:0c:29:87:c0:5d
inet addr:10.1.1.133 Bcast:10.1.1.255 Mask:255.255.255.0
inet6 addr: fe80::20c:29ff:fe87:c05d/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:300 errors:0 dropped:0 overruns:0 frame:0
TX packets:68 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:32228 (31.4 KiB) TX bytes:4072 (3.9 KiB)
Interrupt:19 Base address:0x2000

eth1 Link encap:Ethernet HWaddr 00:0c:29:87:c0:67
inet addr:10.1.1.109 Bcast:10.1.1.255 Mask:255.255.255.0
inet6 addr: fe80::20c:29ff:fe87:c067/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:5285 errors:0 dropped:0 overruns:0 frame:0
TX packets:3319 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:7677393 (7.3 MiB) TX bytes:248631 (242.8 KiB)
Interrupt:19 Base address:0x2080


Destination Gateway Genmask Flags Metric Ref Use Iface
10.1.1.0 * 255.255.255.0 U 0 0 0 eth1
10.1.1.0 * 255.255.255.0 U 0 0 0 eth0
default blackbox.xxxx 0.0.0.0 UG 0 0 0 eth0
default blackbox.xxxx 0.0.0.0 UG 0 0 0 eth1

OS X 10.6.4
en0: flags=8963<UP,BROADCAST,SMART,RUNNING,PROMISC,SIMP LEX,MULTICAST> mtu 1500
ether 00:1f:5b:32:37:88
inet6 fe80::21f:5bff:fe32:3788%en0 prefixlen 64 scopeid 0x4
inet 10.1.1.100 netmask 0xffffff00 broadcast 10.1.1.255
media: autoselect (1000baseT <full-duplex,flow-control>)
status: active
en1: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULT ICAST> mtu 1500
ether 00:1f:5b:32:37:89
inet6 fe80::21f:5bff:fe32:3789%en1 prefixlen 64 scopeid 0x5
inet 10.1.1.157 netmask 0xffffff00 broadcast 10.1.1.255
media: autoselect (1000baseT <full-duplex,flow-control>)
status: active
netstat -nr
Routing tables

Internet:
Destination Gateway Flags Refs Use Netif Expire
default 10.1.1.1 UGSc 12 0 en0
default 10.1.1.1 UGScI 0 0 en1
10.1.1/24 link#4 UCS 13 0 en0
10.1.1/24 link#5 UCSI 1 0 en1
10.1.1.1 0:18:f8:77:a0:3f UHLWI 6 74 en0 1167
10.1.1.11 0:1a:92:b9:e2:1 UHLWI 1 6 en0 1169
10.1.1.14 link#4 UHLWI 0 33 en0
10.1.1.15 0:30:18:ad:ed:ec UHLWI 0 269 en0 1125
10.1.1.16 0:30:18:ad:ed:ec UHLWI 0 78 en0 1128
10.1.1.21 0:c0:48:26:27:5e UHLWI 0 0 en0 1171
10.1.1.21 0:c0:48:26:27:5e UHLWI 0 0 en1 1171
10.1.1.100 127.0.0.1 UHS 0 0 lo0
10.1.1.148 0:c:29:3e:40:c5 UHLWI 0 10 en0 1137
10.1.1.157 127.0.0.1 UHS 0 1 lo0
10.1.1.255 ff:ff:ff:ff:ff:ff UHLWbI 0 15 en0
127 127.0.0.1 UCS 0 0 lo0
127.0.0.1 127.0.0.1 UH 1 14 lo0

Open Solaris 9
ifconfig -a
lo0: flags=2001000849<UP,LOOPBACK,RUNNING,MULTICAST,IPv 4,VIRTUAL> mtu 8232 index 1
inet 127.0.0.1 netmask ff000000
pcn0: flags=1004843<UP,BROADCAST,RUNNING,MULTICAST,DHCP, IPv4> mtu 1500 index 2
inet 10.1.1.148 netmask ffffff00 broadcast 10.1.1.255
pcn1: flags=1004843<UP,BROADCAST,RUNNING,MULTICAST,DHCP, IPv4> mtu 1500 index 3
inet 10.1.1.125 netmask ffffff00 broadcast 10.1.1.255
lo0: flags=2002000849<UP,LOOPBACK,RUNNING,MULTICAST,IPv 6,VIRTUAL> mtu 8252 index 1
inet6 ::1/128

Routing Table: IPv4
Destination Gateway Flags Ref Use Interface
-------------------- -------------------- ----- ----- ---------- ---------
default 10.1.1.1 UG 1 2 pcn0
default 10.1.1.1 UG 1 1 pcn1
10.1.1.0 10.1.1.148 U 1 2 pcn0
10.1.1.0 10.1.1.125 U 1 1 pcn1
127.0.0.1 127.0.0.1 UH 1 28 lo0

Routing Table: IPv6
Destination/Mask Gateway Flags Ref Use If
--------------------------- --------------------------- ----- --- ------- -----
::1 ::1 UH 1 0 lo0


10.04
eth0 Link encap:Ethernet HWaddr 00:0c:29:30:6f:8f
inet addr:10.1.1.139 Bcast:10.1.1.255 Mask:255.255.255.0
inet6 addr: fe80::20c:29ff:fe30:6f8f/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:258 errors:0 dropped:0 overruns:0 frame:0
TX packets:8 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:19342 (19.3 KB) TX bytes:1152 (1.1 KB)

eth1 Link encap:Ethernet HWaddr 00:0c:29:30:6f:99
inet addr:10.1.1.115 Bcast:10.1.1.255 Mask:255.255.255.0
inet6 addr: fe80::20c:29ff:fe30:6f99/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:1038 errors:0 dropped:0 overruns:0 frame:0
TX packets:546 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:86839 (86.8 KB) TX bytes:157150 (157.1 KB)

Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
10.1.1.0 * 255.255.255.0 U 0 0 0 eth1
10.1.1.0 * 255.255.255.0 U 0 0 0 eth0
default blackbox.xxxx 0.0.0.0 UG 100 0 0 eth0
default blackbox.xxxx 0.0.0.0 UG 100 0 0 eth1