Can anybody help?

Sorry for stupid question but...

The problem is:

I
have local network with IP 192.168.1.0/24 and one of the computers in this
network, let's say 192.168.1.100 is the gateway to Internet.

There are two modems are plugged to this computer and connected to my ISP.

ppp0 - 212.x.x.1
ppp1 - 212.x.x.2

All
I need to do is to setup system on the gateway in the way they will give
internet access for local IPs 192.168.1.1-10 throug ppp0 and for local IPs
192.168.1.50-70 throug ppp0?

It seems like it's easy but I have no idea HOW?

PS: sorry for my English...