I'd like to use my android phone internet connection (and, if possible, also my girlfriend's) to speed up my weak adsl (we live on a mountain top). Is there any tool to achieve this result on any operating system?
Not easily. You would need a multiWAN router in order to handle the routing and you would need a computer to connect each phone to that would share the connection out into one of the WAN ports. It would be an awkward setup, would require two dedicated machines to do nothing else but relay the connection from the phones, would require a very expensive router (or a third heavily configured computer with multiple NICs) to get this up and running, and to be honest, you wouldn't get much benefit. The type of connection it would provide would be load balanced, so it wouldn't be a combined speed. This means that if you had a 1.5 Mbps ADSL, 1.5 Mbps phone and another 1.5 Mbps phone, your speed would never be over 1.5 Mbps, but you could make 3 simultaneous 1.5 Mbps connections when needed, the router would balance the loads based on use on the network.