Does this make any sense??

Here is my question…

I have 1 proxy server 192.168.1.100, and 2 eth cards eth0 and eth1 both are connected to a switch.

I have 2 gateways connected to my switch (2 ADSL modems 192.168.1.1 and 192.168.1.2 – 2 different ISPs)…I have 18 PC connected to the same switch…

I want 9 PCs (192.168.1.50-58) to use the 1st gateway 192.168.1.1 and the other 9 (192.168.1.60-68) to use the 2nd gateway 192.168.1.2, and all the PCs use the same proxy 192.168.1.100. Both gateways are defined in my proxy server….

Is it possible to use the iptable rules to do such thing??? I am not trying to merge 2 lines, I am just trying to force some PCs (based on the IP addresses) to use certain gateway…

*EDIT*
So basically what i was is this

if IP == 192.168.1.50-58 use getway 1
if IP == 192.168.1.60-68 use getway 2