It was a fantasy of mine, back when cable ruled and DSL was a distant dream.
It's also known as
Inverse Multiplexing.
You will need an endpoint outside your network, to recombine the signal into one. For example on a VPS (not VPN) somewhere outside Lebanon, with a 100mbps connection to the internet.
Another option is load balancing, so you have two DSL connections and the sessions get distributed across the two. It will result in a different type of performance gain
There are solutions to do that and I have seen someone implement that in Lebanon.
For inverse multiplexing, to go back to the original topic, I am not too familiar with any solutions. I know that you can do many things with Linux, and with some work and time, it sounds possible to set up such a system.
That's roughly the extent of my knowledge on the topic.