Double ethernet interfaces problem

["Rodrigo Haces" <rodrigo@haces.com.mx>]

Hi, i just installed debian sarge 3.1 on my pc, i have 2 ethernet interfaces,
eth0 and eth1, im going to configure a firewall with this, but by now im just
testing, but i have the next problem

my eth0 and main interface has the ip 192.168.0.100
my eth1 has by the testing period the ip 192.168.0.99.

both eth are connected to the same switch with the same local network, my eth0
is on all the time, so it works fine, but when i bring up th eth1 i cant bring
it down anymore, i do ifdown eth0 and ping 192.168.0.99 and can still connect
to this ip with ssh, ping, etc. if then i ifdown eth0 both ips die.

So what can be the problem? te problem can be in that both interfaces are at
the same switch at the same network with the same iprange and netmask? or is
there a problem with ifdown/up 

Thanks
Rodrigo