So far there have only been two times the Borg sent a cube to Earth for assimilation, and both times were defeated by means that were at the time very out of the blue (Data hacking the collective via Locutus, de-borged Picard still having a certain connection that allows him to find a cube's weak point).
I think after two times the Borg will have learned not to take any more chances and send more than one. That's assuming they really think Earth is that vital enough to send two, otherwise they'll just let it be until new developments turn up that changes their mind.
I think after two times the Borg will have learned not to take any more chances and send more than one. That's assuming they really think Earth is that vital enough to send two, otherwise they'll just let it be until new developments turn up that changes their mind.