On the OpenWRT MultiWAN package is done once or twice. I also have two ISP and switching to openwrt is not planned, is dd-wrt, like:
Once you have one cable, then just lift the connection on the same vlanе, only through the point write a number, such as vlan1.1 with the new parameters.
Next, make masquerading and create the ip rule for both interfaces(otherwise, the incoming packets outgoing will not go via that interface, through which came, and in default), requires that your put on them and be sure to turn off rp_filter on the interface.
And in openwrt this is done by simply writing the config.