in one interface out the other

I have two Tomato Routers, each with their separate WANs for example x.x.x.1 and x.x.x.2
I want traffic coming in to x.x.x.1 and pass to the other router and go out through x.x.x.2

Any help is much appreciated. I noticed if i disable spoofing and /proc/sys/net/ipv4/conf/all/rp_filter it could work, but I want to be able to allow just specific source IPs and on specific ports to be able to do that.

Thanks in advance