What you need is a Load Balancing Router with 2 wan ports
Edimax 2 WAN + 4LAN Load Balancing Router
Smart Load Balancing Three load balancing types allow administrators to manage the load balancing methods best suited to the LAN environment and to maximize bandwidth across different connections.
High availability with double bandwidth For small-to-medium size business, Load Balancing is the ideal product.
With Load Balancer, up to 253 users are connected to the Internet with Double Broadband Speed.
Load Balancing supports QoS for controlling traffic priority.
With QoS, businesses can make the critical traffic or application at high priority and route first.
WAN port Health Check to against connection fail
Load Balancing offers automatic outbound traffic backup to against the failure of either broadband connection to protect your business.