First, only business class routers/firewalls provide this ability. Not cheap.
Second, the second internet connection is only useful as a backup for the first. Routers can't use both connections simutaniously.
You should drop one service and upgrade the other if you want more bandwodth.
If for some reason you're stuck with two internet services, you could get a second router and connect half of your computers to each router. That would spread out your usage. The problem there is that the seven computers wouldn't all be able to talk to eachother. You could do some voodoo with a vpn tunnel between the two routers (if they're capable) but that starts making things complicated with static routes and sub-domains.