I think what you are looking for is a load balancing script.
i'm not sure if its possible to do it with wireless connections but i do know it possible to do with 2 Internet connections going directly to Ethernet.
And the effect of a 1mb and a 1.5mb would not give you a 2.5mb connection.
Plus things like torrents etc have difficulty in using 2 different ip's.
all this could be wrong tho lol.