In the vast majority of cases, there is no way to set up the system to do what you're trying to do.
Give up. It's hopeless without a semi-major infusion of cash.
Unless you have "Server" NICs that support "teaming," and a compatible switch to plug 'em into, it won't work.
The bandwidth out of your LAN (LAN-->WAN) probably wouldn't support the extra bandwidth anyway.
FWIW
Scott