First of all you cannot have two interfaces on the same router be part of the same IP subnet. Without knowing the details about your setup it is hard to suggest anything so it will be very helpful if you can explain how your remote sites are connecting to the central site and what is the LAN/WAN infrastrcuture at your central site.
That being said what is the bandwidth of your internet connection? More than likely it is the internet link that is the bottleneck and not the router's ethernet interface but again we need more details of your setup. Are you actually experiencing heavy loads on this ethernet interface or you just want to use both as you think it will be a better way to do things?