First, you will need to contact ISP and see if they support link bonding. If they do not you will never get both aggregate speeds at once but you can still do a dual WAN load balanced setup. You will probably have to get your ISP to bridge both DSL modems and then you will need a third router capable of dual WAN. Once you have the parts and the correct configuration on the ISP side you just set up the router in dual WAN mode based on manufacturer setup. If the connections are 2 different ISP's then the only available setup is load balance.