a small update. on ubuntu 12.04 lts 3.2.0-23-generic x86_64 it seems that in the scenario that was mentioned my setup work fine with tproxy and masquerading.so the mentioned setup is ok but only with squid tproxy runing else the packets will be identified by routing hash memory and will routed without masquerading.
Eliezer -- Eliezer Croitoru https://www1.ngtech.co.il IT consulting for Nonprofit organizations eliezer <at> ngtech.co.il