You'd have to connect 2nd internet to main router to any LAN socket, enable dual WAN feature with load balance mode and select that LAN socket as secondary WAN.

If for some reason you cannot do so you could just add second router to your network with different IP and use it as second gateway, but you'd have to set it manually on each pc in network adapter settings.
I don't know if there's another way in Asus wrt.