You need a router that can do "load balancing," Mikrotik or Ubiquity make routers that can do this but it will take some networking skills to accomplish this. (you can probably even do it with a home wifi router flashed with tomato, openwrt or ddwrt)
I am sure there are other dual wan routers out there but I am not familiar with them.
search for dual wan + openwrt or ddwrt or tomato
I am sure there are other dual wan routers out there but I am not familiar with them.
search for dual wan + openwrt or ddwrt or tomato