Load balancing comes in a variety of flavours. In all cases, though, a single transfer (like a single file download from a web browser) can never go faster than one of the load balanced connections. The only time you'll see the full throughput is when you have multiple simultaneous transfers. This tends to happen most easily with software like BitTorrent, which naturally splits files up into many simultaneous transfers.
Managing this sort of thing can be difficult. If it's just two identical connections, it's not so hard; you can get away with just balancing connections evenly. Maybe even take actual load into account; if connection 1 is maxed out with a single connection, and connection 2 is at half capacity with ten connections, the logical thing to do is to put more load on connection 2, not 1.
If the connections aren't identical, though, that gets a lot harder. For one thing, how much capacity does a connection actually have? There's no easy way to measure this, for a piece of software to know how fast your connections are. Should it be the fastest speed they've seen over a given connection? Well, what if the conditions change, and that speed isn't accurate anymore? Figuring this sort of thing out on the fly is a very non-trivial problem, which is why most load-balancing solutions require the user to manually enter the speeds of the various connections.
But when you're mixing and matching different kinds of connections like satellite and cellular, now you're even combining things with completely different properties in terms of things like latency and reliability... I'm not sure there's really any software on the market right now that will work really well doing that. There's some on the horizon, though. Connectify Dispatch got some hype recently, since Connectify claims that it manages all this stuff between disparate connections on the fly, dynamically picking what to use:
It's not out yet, but I'm interested in seeing reviews when it is. It's not going to be as good as MLPPP on two identical connections, but for disparate connections where load balancing is the best you can do, it looks like it will be a huge improvement over the current stuff out there.--
Developer: Tomato/MLPPP, Linux/MLPPP, etc »fixppp.org