The problem you'd have with bonding two Three 3G connections to the same mast is that you'll be sharing your own bandwidth e.g. achieving nothing.
Even if you did bond two 3G connections from two separate carriers, the improvement might not be what you had hoped for due to the latency inherent to 3G.
Our situation is similar to yours albeit we can get ADSL but it's too slow to bother with and we have Three 3G, have had it for 2 years.
Have looked at other options here but our best bet is presently to either pay for bonded ADSL which costs a fortune and I'd need four or five lines to get decent speeds, or wait untii the local cell is upgraded to 7.2Mbps - Three aim to have them all done by end 2011.
There's also the option of satellite (news, front page of site) but latency is high and usage limits uncertain.