Policy routing with 2 x WAN with one default gateway

Hi friends,
I have a question about following situation…
My RB2011 has 2 x WAN ports which are configured with dynamic IP addresses from my ISP. So WAN1 has address x.x.65.3/20 and WAN2 has x.x.65.9/20, i.e they belong to the same subnet - x.x.64.0/20. The default gateway for the both of WAN1 and WAN2 is x.x.64.1. My local lan is 192.168.1.0/24.
I want to masquerade the part of IP addresses from my local LAN through WAN1 and other part of IP’s through WAN2.
How is it possible to do this? Thanks in advance!

First you have to set proper routing; to be able to have two default routes to the same gateway through different interfaces, you do this by specifying the interface in the gateway portion on ip > routes, i.e.

gateway=x.x.64.1%wan1

and

gateway=x.x.64.1%wan2

Thank you for the replay…
I did this, but how to deal with routing?

see http://forum.mikrotik.com/t/selective-wan-port-routing-confused/96371/1