scope,
using the unix configurations for IP routing, something should be possible whereby you could take advantage of both connections to maximise bandwidth. I don't think it's straight forward to combine both connections and get 1Mbit/s from a single IP address source, but it should be possible to improve your potential bandwidth by having different IP source addresses communicate with you via different airport connections.
This is only theoretical, but I'd see if you can make sense of this idea and maybe come up with a working solution:
setup both airport networks so they act as if they are part of the same subnet (ie, if using static IP, give your mac and the 2 basestations IP addresses on the same subnet eg 192.168.1.xxx).
Use the unix
route command to setup a routing table for your mac that maps different IP addresses to the different base stations ie:
IP 191.0.0.0 Mask 191.0.0.0 Gateway (IP address of 1st basestation) en1
IP 0.0.0.0 Mask 0.0.0.0 Gateway (IP address of 2nd basestation) en1
The idea here is that IP addresses higher than 190.255.255.255 get sent via one airport basestation, and addresses lower than 190.255.255.255 get sent via the other basestation. You should be able then to use the full 1Mbit/s by accessing IP addresses on both sides of the cutoff point.
The only stumbling block here that I can see is how Airport handles a 2 basestation setup like this - I understand this should work just like a roaming network setup, but I have never seen a network setup like this to test it on.
Of course this type of bandwidth sharing would be much easier if you had each connection on different interfaces - either (simply) by having your own cable connection hooked in via ethernet (en0) and your neighbour's over airport (en1), or (more difficult) manage to convince OS X that the airport card is actually 2 interfaces (en1 and en2).
Good luck with your little challenge - let me know how you go!
Cheers,
James Hodge
james.hodge@beatentrack.com.au