routing + 2 NICs on teh same network

[Chavdar Videff <chavdar@mr-bricolage.bg>]

Hello,

We have the following problem with the routing.
We have the following structure:
Internet   >>>>  Real Ip address on Cisco Router >>>> Internal ip address on 
Cisco Router (10.50.40.31) >>>>> LAN

In the LAN space we deploy a Debian Linux gateway and firewall to be in 
between Cisco Router and LAN. We noticed the following problem:

On Debian gateway:
eth0 - 10.50.40.28
eth1 - 10.50.40.29

10.50.40.29 is the gateway IP address for all hosts on the LAN.
However if deployed with above settings - there is no routing possible.
If we assign the 10.50.40.29 IP address to eth0:1 everything works as 
expected.

If eth1 is another network (10.50.41.29 , again for hosts in that network 
everything is OK).

My question is:
Why isn't it possible to have 2 NICs on same network and have routing achieved 
or, if it is possible, how is it accomplished.

Regards,

Chavdar Videff