I had this configured on dd-wrt, openwrt did not do, but everything is solved either miltiwan, or just your hands. First you need to make a separate Vilan to connect a second. Through webfac, you can use either nvram, robocfg.
That's how I
Ie one provider stick in the WAN port, the other to port 4.
In openwrt the entire network is configured in the file /etc/config/network. You can do what you want, no package multiwan. Just keep in mind, you have like NAT with two sides, you will need to mark the coming(!) the packages from each provider, and to send a response also through this provider, and they will go through the default provider.
Learn more here www.dd-wrt.com/wiki/index.php/Dual-WAN_for_simple_round-robin_load_equalization
I'm only there half to read this, because I round Robin was needed.