Can't set default gateway to different subnet

[redss]

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What am I doing wrong here...
This is slackware 12. I'm trying to share my internet connection (on eth0) with a computer connected to eth1.

eth0 is 192.168.0.101, eth1 is 192.168.1.100 (different subnets).

But when I try to make eth1's default gateway the same as eth0's, see the error I get:

# route add default gw 192.168.0.1 dev eth1
SIOCADDRT: No such process

This is supposed to work, per http colon slash slash lindesk.com/2007/04/internet-connection-sharing-using-iptables/

more informational output follows....

# ifconfig
eth0      Link encap:Ethernet  HWaddr 00:07:E9:5C:77:81  
          inet addr:192.168.0.101  Bcast:192.168.0.255  Mask:255.255.255.0
          UP BROADCAST NOTRAILERS RUNNING MULTICAST  MTU:1500  Metric:1

eth1      Link encap:Ethernet  HWaddr 00:08:C7:E6:F2:04  
          inet addr:192.168.1.100  Bcast:192.168.1.255  Mask:255.255.255.0
          UP BROADCAST NOTRAILERS RUNNING MULTICAST  MTU:1500  Metric:1

# route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.1.0     *               255.255.255.0   U     0      0        0 eth1
192.168.0.0     *               255.255.255.0   U     0      0        0 eth0
loopback        *               255.0.0.0       U     0      0        0 lo
default         192.168.0.1     0.0.0.0         UG    0      0        0 eth0

[Kloschüssel]

As often google is really helpful:

How to setup the PC as the Router ? - FedoraForum.org

I blindly searched "howto configure computer as router". Your command above wouldn't work because it would add a second default route and one cannot have two default routes - think of it and you will understand why.