2 Cable Accounts 2 Routers

hi i have a game server im running right now for a client and he has recently got another cable modem from his cable provider and it works fine.

So he has 2 cable modems 2 accounts
he also has 2 router both linksys
2 Nics in the server

now if i unplug one nic the internet still works fine because it used the other one what we want to do is combine the speeds

When we do a bandwidth test it comes back at 1.5 down and 300 up so its only using one connect
we want it to use both so we would get 3.0 down and 600 up

Please tell me what im doing wrong here he doesnt have any more money to buy a dual wan router he wants to use this equipment or maybe there is a program i can use to do this

Thanks