Load balancing and broadband bonding are two different technologies. With load balancing you will not get the sum of the speed like you are trying to.
Similarly, the modems that support MLPPP based bonding is no good, as almost no ISP in the world supports MLPPP bonding.

You can accomplish broadband bonding with a device like truffle from mushroom networks. They also have software version of that I believe. That will give you the speed of all your ISPs. We have deployed this for many of our clients and has been extremely successful for us. Literally, the network related problems disappeared. So now we deploy the truffle units in every new install that we have.