There are at least two ways I've heard to accomplish this in linux, the first way I know is using a patched ifconfig that does aliases, the second way is described below. Note that, for the 2.0.x kernels, things have gotten much simpler. How to setup multiple IP's on later kernels is described in the IP Alias mini-HOWTO.

 Using Proxy ARP 


insmod -o dummy0 dummy.o        Install the dummy module which needs compiling
                                first (make modules under /usr/src/linux)


ifconfig dummy0 1.2.3.4         Whatever IP address you want it to have.
                                This will be ref. as the second host


route add 1.2.3.4 dummy0        Set the static route to it


arp -s 1.2.3.4 00:00.... pub    Replace the last bit with the ETHERNET address
                                of your eth0

Basically your eth0 then advertises by arp that it can take IP packets 
for 1.2.3.4 and seeing as it has the route.......

Setup the DNS etc... and your away.  If you want another dummy device 
you do:

insmod -o dummy1 dummy.o

then repeat the above procedure using dummy1