From a reader’s comment to the TUAW article, Worldwide Mac: getting online in New Zealand:

What I’d recommend doing to alleviate your problem is to set up some kind of home server (perhaps a Mac Mini) and configure an Apache / Squid proxy server on it. Set up each Mac to use the server as its proxy, and you’ll notice a significant reduction in bandwidth, as well as a significant increase in speed, because you’ll pull down duplicate resources via your home network rather than the slow internet link.

With a caching proxy (and a sufficiently large cache, a few gigabytes or so) you should be able to, for instance, download software updates once and then have the other Mac fetch them from the cache